Represent bucket as object instead of []byte name.

Thanks to this change:
  - all the maps bucket -> buffer are indexed by int's instead of
string. No need to do: byte[] -> string -> hash conversion on each
access.
  - buckets are strongly typed in backend/mvcc API.

server/mvcc/backend/hooks_test.go

@@ -22,10 +22,11 @@ import (
 	"github.com/stretchr/testify/assert"
 	"go.etcd.io/etcd/server/v3/mvcc/backend"
 	betesting "go.etcd.io/etcd/server/v3/mvcc/backend/testing"
+	"go.etcd.io/etcd/server/v3/mvcc/buckets"
 )
 
 var (
-	bucket = []byte("bucket")
+	bucket = buckets.Test
 	key    = []byte("key")
 )