server: Extract hasher to separate interface

Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>

server/mvcc/kv.go

@@ -112,7 +112,6 @@ const (
 type KV interface {
 	ReadView
 	WriteView
-	Hasher
 
 	// Read creates a read transaction.
 	Read(mode ReadTxMode, trace *traceutil.Trace) TxnRead
@@ -120,6 +119,9 @@ type KV interface {
 	// Write creates a write transaction.
 	Write(trace *traceutil.Trace) TxnWrite
 
+	// HashStorage returns HashStorage interface for KV storage.
+	HashStorage() HashStorage
+
 	// Compact frees all superseded keys with revisions less than rev.
 	Compact(trace *traceutil.Trace, rev int64) (<-chan struct{}, error)
 
@@ -131,14 +133,6 @@ type KV interface {
 	Close() error
 }
 
-type Hasher interface {
-	// Hash computes the hash of the KV's backend.
-	Hash() (hash uint32, revision int64, err error)
-
-	// HashByRev computes the hash of all MVCC revisions up to a given revision.
-	HashByRev(rev int64) (hash KeyValueHash, revision int64, err error)
-}
-
 // WatchableKV is a KV that can be watched.
 type WatchableKV interface {
 	KV