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/buckets/bucket.go

@@ -0,0 +1,80 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package buckets
+
+import (
+	"bytes"
+
+	"go.etcd.io/etcd/server/v3/mvcc/backend"
+)
+
+var (
+	keyBucketName   = []byte("key")
+	metaBucketName  = []byte("meta")
+	leaseBucketName = []byte("lease")
+	alarmBucketName = []byte("alarm")
+
+	clusterBucketName = []byte("cluster")
+
+	membersBucketName        = []byte("members")
+	membersRemovedBucketName = []byte("members_removed")
+
+	authBucketName      = []byte("auth")
+	authUsersBucketName = []byte("authUsers")
+	authRolesBucketName = []byte("authRoles")
+
+	testBucketName = []byte("test")
+)
+
+var (
+	Key     = backend.Bucket(bucket{id: 1, name: keyBucketName, safeRangeBucket: true})
+	Meta    = backend.Bucket(bucket{id: 2, name: metaBucketName, safeRangeBucket: false})
+	Lease   = backend.Bucket(bucket{id: 3, name: leaseBucketName, safeRangeBucket: false})
+	Alarm   = backend.Bucket(bucket{id: 4, name: alarmBucketName, safeRangeBucket: false})
+	Cluster = backend.Bucket(bucket{id: 5, name: clusterBucketName, safeRangeBucket: false})
+
+	Members        = backend.Bucket(bucket{id: 10, name: membersBucketName, safeRangeBucket: false})
+	MembersRemoved = backend.Bucket(bucket{id: 11, name: membersRemovedBucketName, safeRangeBucket: false})
+
+	Auth      = backend.Bucket(bucket{id: 20, name: authBucketName, safeRangeBucket: false})
+	AuthUsers = backend.Bucket(bucket{id: 21, name: authUsersBucketName, safeRangeBucket: false})
+	AuthRoles = backend.Bucket(bucket{id: 22, name: authRolesBucketName, safeRangeBucket: false})
+
+	Test = backend.Bucket(bucket{id: 100, name: testBucketName, safeRangeBucket: false})
+)
+
+type bucket struct {
+	id              backend.BucketID
+	name            []byte
+	safeRangeBucket bool
+}
+
+func (b bucket) ID() backend.BucketID    { return b.id }
+func (b bucket) Name() []byte            { return b.name }
+func (b bucket) String() string          { return string(b.Name()) }
+func (b bucket) IsSafeRangeBucket() bool { return b.safeRangeBucket }
+
+var (
+	MetaConsistentIndexKeyName = []byte("consistent_index")
+	MetaTermKeyName            = []byte("term")
+)
+
+// DefaultIgnores defines buckets & keys to ignore in hash checking.
+func DefaultIgnores(bucket, key []byte) bool {
+	// consistent index & term might be changed due to v2 internal sync, which
+	// is not controllable by the user.
+	return bytes.Compare(bucket, Meta.Name()) == 0 &&
+		(bytes.Compare(key, MetaTermKeyName) == 0 || bytes.Compare(key, MetaConsistentIndexKeyName) == 0)
+}