add interface for MerkleDB (ava-labs#1519)

Author: Dan Laine

scripts/mocks.mockgen.txt

@@ -48,4 +48,5 @@ github.com/ava-labs/avalanchego/vms/registry=VMGetter=vms/registry/mock_vm_gette
 github.com/ava-labs/avalanchego/vms/registry=VMRegisterer=vms/registry/mock_vm_registerer.go
 github.com/ava-labs/avalanchego/vms/registry=VMRegistry=vms/registry/mock_vm_registry.go
 github.com/ava-labs/avalanchego/vms=Factory,Manager=vms/mock_manager.go
-github.com/ava-labs/avalanchego/x/sync=Client=x/sync/mock_client.go
+github.com/ava-labs/avalanchego/x/merkledb=MerkleDB=x/merkledb/mock_db.go
+github.com/ava-labs/avalanchego/x/sync=Client=x/sync/mock_client.go

x/merkledb/batch.go

@@ -12,7 +12,7 @@ var _ database.Batch = (*batch)(nil)
 type batch struct {
 	database.BatchOps
 
-	db *Database
+	db *merkleDB
 }
 
 // apply all operations in order to the database and write the result to disk