Implement base driver.Connection type

Implements the topology.connection and topology.Connection types.

GODRIVER-930

Change-Id: If3c133b754028b8e832aa198b285593f6f2efbfc

x/mongo/driver/driver.go

@@ -3,6 +3,7 @@ package driver // import "go.mongodb.org/mongo-driver/x/mongo/driver"
 import (
 	"context"
 
+	"go.mongodb.org/mongo-driver/x/network/address"
 	"go.mongodb.org/mongo-driver/x/network/description"
 )
 
@@ -25,4 +26,12 @@ type Connection interface {
 	Description() description.Server
 	Close() error
 	ID() string
+	Address() address.Address
+}
+
+// ErrorProcessor implementations can handle processing errors, which may modify their internal state.
+// If this type is implemented by a Server, then Operation.Execute will call it's ProcessError
+// method after it decodes a wire message.
+type ErrorProcessor interface {
+	ProcessError(error)
 }

x/mongo/driverlegacy/topology/DESIGN.md

@@ -0,0 +1,16 @@
+# Topology Package Design
+This document outlines the design for this package.
+
+## Connection
+Connections are handled by two main types and an auxiliary type. The two main types are `connection`
+and `Connection`. The first holds most of the logic required to actually read and write wire
+messages. Instances can be created with the `newConnection` method. Inside the `newConnection`
+method the auxiliary type, `initConnection` is used to perform the connection handshake. This is
+required because the `connection` type does not fully implement `driver.Connection` which is
+required during handshaking. The `Connection` type is what is actually returned to a consumer of the
+`topology` package. This type does implement the `driver.Connection` type, holds a reference to a
+`connection` instance, and exists mainly to prevent accidental continued usage of a connection after
+closing it.
+
+The connection implementations in this package are conduits for wire messages but they have no
+ability to encode, decode, or validate wire messages. That must be handled by consumers.