mvcc: improve the use of locks in index.go
mvcc: improve the use of locks in index.go

To make the meaning of the RangeSince function in treeIndex clearer.

ajityagaty/etcd,etcd-io/etcd,mqliang/etcd,ajityagaty/etcd,etcd-io/etcd,mqliang/etcd,ajityagaty/etcd,mqliang/etcd,etcd-io/etcd