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/backend_bench_test.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	betesting "go.etcd.io/etcd/server/v3/mvcc/backend/testing"
+	"go.etcd.io/etcd/server/v3/mvcc/buckets"
 )
 
 func BenchmarkBackendPut(b *testing.B) {
@@ -41,13 +42,13 @@ func BenchmarkBackendPut(b *testing.B) {
 	batchTx := backend.BatchTx()
 
 	batchTx.Lock()
-	batchTx.UnsafeCreateBucket([]byte("test"))
+	batchTx.UnsafeCreateBucket(buckets.Test)
 	batchTx.Unlock()
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		batchTx.Lock()
-		batchTx.UnsafePut([]byte("test"), keys[i], value)
+		batchTx.UnsafePut(buckets.Test, keys[i], value)
 		batchTx.Unlock()
 	}
 }