mvcc: Remove unused revisions and change comment rev to modified

Signed-off-by: Hongfei Huang <853885165@qq.com>

server/storage/mvcc/key_index.go

@@ -37,7 +37,7 @@ var (
 // For example: put(1.0);put(2.0);tombstone(3.0);put(4.0);tombstone(5.0) on key "foo"
 // generate a keyIndex:
 // key:     "foo"
-// rev: 5
+// modified: 5
 // generations:
 //    {empty}
 //    {4.0, 5.0(t)}

server/storage/mvcc/key_index_test.go

@@ -24,7 +24,7 @@ import (
 
 func TestKeyIndexGet(t *testing.T) {
 	// key: "foo"
-	// rev: 16
+	// modified: 16
 	// generations:
 	//    {empty}
 	//    {{14, 0}[1], {14, 1}[2], {16, 0}(t)[3]}
@@ -680,7 +680,7 @@ func TestGenerationWalk(t *testing.T) {
 
 func newTestKeyIndex(lg *zap.Logger) *keyIndex {
 	// key: "foo"
-	// rev: 16
+	// modified: 16
 	// generations:
 	//    {empty}
 	//    {{14, 0}[1], {14, 1}[2], {16, 0}(t)[3]}

server/storage/mvcc/revision.go

@@ -59,9 +59,3 @@ func bytesToRev(bytes []byte) revision {
 		sub:  int64(binary.BigEndian.Uint64(bytes[9:])),
 	}
 }
-
-type revisions []revision
-
-func (a revisions) Len() int           { return len(a) }
-func (a revisions) Less(i, j int) bool { return a[j].GreaterThan(a[i]) }
-func (a revisions) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }