Before this patch, the tombstone can be deleted if its revision is equal
compacted revision. It causes that the watch subscriber won't get this
DELETE event. Based on Compact API[1], we should keep tombstone revision
if it's not less than the compaction revision.
> CompactionRequest compacts the key-value store up to a given revision.
> All superseded keys with a revision less than the compaction revision
> will be removed.
[1]: https://etcd.io/docs/latest/dev-guide/api_reference_v3/
Signed-off-by: Wei Fu <fuweid89@gmail.com>
