feat: ADR 040: Implement in-memory DB backend (#9952)

## Description

Implements an in-memory backend for the DB interface introduced by #9573 and specified by [ADR-040](https://github.com/cosmos/cosmos-sdk/blob/eb7d939f86c6cd7b4218492364cdda3f649f06b5/docs/architecture/adr-040-storage-and-smt-state-commitments.md). This expands on the [btree](https://pkg.go.dev/github.com/google/btree)-based [`MemDB`](https://github.com/tendermint/tm-db/tree/master/memdb) from `tm-db` by using copy-on-write clones to implement versioning.

Resolves: vulcanize#2

Will move out of draft once #9573 is merged and rebased on.

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) n/a
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -139,6 +139,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (errors) [\#8845](https://github.com/cosmos/cosmos-sdk/pull/8845) Add `Error.Wrap` handy method
 * [\#8518](https://github.com/cosmos/cosmos-sdk/pull/8518) Help users of multisig wallets debug signature issues.
 * [\#9573](https://github.com/cosmos/cosmos-sdk/pull/9573) ADR 040 implementation: New DB interface
+* [\#9952](https://github.com/cosmos/cosmos-sdk/pull/9952) ADR 040: Implement in-memory DB backend
 
 
 ### Client Breaking Changes

db/README.md

@@ -0,0 +1,39 @@
+# Key-Value Database
+
+Databases supporting mappings of arbitrary byte sequences.
+
+## Interfaces
+
+The database interface types consist of objects to encapsulate the singular connection to the DB, transactions being made to it, historical version state, and iteration.
+
+### `DBConnection`
+
+This interface represents a connection to a versioned key-value database. All versioning operations are performed using methods on this type.
+  * The `Versions` method returns a `VersionSet` which represents an immutable view of the version history at the current state.
+  * Version history is modified via the `{Save,Delete}Version` methods.
+  * Operations on version history do not modify any database contents.
+
+### `DBReader`, `DBWriter`, and `DBReadWriter`
+
+These types represent transactions on the database contents. Their methods provide CRUD operations as well as iteration.
+  * Writeable transactions call `Commit` flushes operations to the source DB.
+  * All open transactions must be closed with `Discard` or `Commit` before a new version can be saved on the source DB.
+  * The maximum number of safely concurrent transactions is dependent on the backend implementation.
+  * A single transaction object is not safe for concurrent use.
+  * Write conflicts on concurrent transactions will cause an error at commit time (optimistic concurrency control).
+
+#### `Iterator`
+
+  * An iterator is invalidated by any writes within its `Domain` to the source transaction while it is open.
+  * An iterator must call `Close` before its source transaction is closed.
+
+### `VersionSet`
+
+This represents a self-contained and immutable view of a database's version history state. It is therefore safe to retain and conccurently access any instance of this object.
+
+## Implementations
+
+### In-memory DB
+
+The in-memory DB in the `db/memdb` package cannot be persisted to disk. It is implemented using the Google [btree](https://pkg.go.dev/github.com/google/btree) library.
+  * This currently does not perform write conflict detection, so it only supports a single open write-transaction at a time. Multiple and concurrent read-transactions are supported.

db/dbtest/benchmark.go

@@ -0,0 +1,109 @@
+package dbtest
+
+import (
+	"bytes"
+	"encoding/binary"
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	dbm "github.com/cosmos/cosmos-sdk/db"
+)
+
+func Int64ToBytes(i int64) []byte {
+	buf := make([]byte, 8)
+	binary.BigEndian.PutUint64(buf, uint64(i))
+	return buf
+}
+
+func BytesToInt64(buf []byte) int64 {
+	return int64(binary.BigEndian.Uint64(buf))
+}
+
+func BenchmarkRangeScans(b *testing.B, db dbm.DBReadWriter, dbSize int64) {
+	b.StopTimer()
+
+	rangeSize := int64(10000)
+	if dbSize < rangeSize {
+		b.Errorf("db size %v cannot be less than range size %v", dbSize, rangeSize)
+	}
+
+	for i := int64(0); i < dbSize; i++ {
+		bytes := Int64ToBytes(i)
+		err := db.Set(bytes, bytes)
+		if err != nil {
+			// require.NoError() is very expensive (according to profiler), so check manually
+			b.Fatal(b, err)
+		}
+	}
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		start := rand.Int63n(dbSize - rangeSize) // nolint: gosec
+		end := start + rangeSize
+		iter, err := db.Iterator(Int64ToBytes(start), Int64ToBytes(end))
+		require.NoError(b, err)
+		count := 0
+		for iter.Next() {
+			count++
+		}
+		iter.Close()
+		require.EqualValues(b, rangeSize, count)
+	}
+}
+
+func BenchmarkRandomReadsWrites(b *testing.B, db dbm.DBReadWriter) {
+	b.StopTimer()
+
+	// create dummy data
+	const numItems = int64(1000000)
+	internal := map[int64]int64{}
+	for i := 0; i < int(numItems); i++ {
+		internal[int64(i)] = int64(0)
+	}
+
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		{
+			idx := rand.Int63n(numItems) // nolint: gosec
+			internal[idx]++
+			val := internal[idx]
+			idxBytes := Int64ToBytes(idx)
+			valBytes := Int64ToBytes(val)
+			err := db.Set(idxBytes, valBytes)
+			if err != nil {
+				// require.NoError() is very expensive (according to profiler), so check manually
+				b.Fatal(b, err)
+			}
+		}
+
+		{
+			idx := rand.Int63n(numItems) // nolint: gosec
+			valExp := internal[idx]
+			idxBytes := Int64ToBytes(idx)
+			valBytes, err := db.Get(idxBytes)
+			if err != nil {
+				b.Fatal(b, err)
+			}
+			if valExp == 0 {
+				if !bytes.Equal(valBytes, nil) {
+					b.Errorf("Expected %v for %v, got %X", nil, idx, valBytes)
+					break
+				}
+			} else {
+				if len(valBytes) != 8 {
+					b.Errorf("Expected length 8 for %v, got %X", idx, valBytes)
+					break
+				}
+				valGot := BytesToInt64(valBytes)
+				if valExp != valGot {
+					b.Errorf("Expected %v for %v, got %v", valExp, idx, valGot)
+					break
+				}
+			}
+		}
+
+	}
+}