Add exchange.SessionExchange interface for exchanges that support sessions.

Blockservice has an explicit dependency on bitswap so it can
call NewSession. It should rely on the exchange interfaces though, not
on specific implementations.

License: MIT
Signed-off-by: Hector Sanjuan <hector@protocol.ai>

interface.go

-// package exchange defines the IPFS exchange interface
+// Package exchange defines the IPFS exchange interface
 package exchange
 
 import (
@@ -10,8 +10,7 @@ import (
 	cid "gx/ipfs/QmcZfnkapfECQGcLZaf9B79NRg7cRa9EnZh4LSbkCzwNvY/go-cid"
 )
 
-// Any type that implements exchange.Interface may be used as an IPFS block
-// exchange protocol.
+// Interface defines the functionality of the IPFS block exchange protocol.
 type Interface interface { // type Exchanger interface
 	Fetcher
 
@@ -30,3 +29,10 @@ type Fetcher interface {
 	GetBlock(context.Context, *cid.Cid) (blocks.Block, error)
 	GetBlocks(context.Context, []*cid.Cid) (<-chan blocks.Block, error)
 }
+
+// SessionExchange is an exchange.Interface which supports
+// sessions.
+type SessionExchange interface {
+	Interface
+	NewSession(context.Context) Interface
+}