Redundant compact most bottom level on manual compact

### Expected behavior
When all data compacted to most bottom level -- at this time point, there is no any other level except the bottom most level.

The manual compaction should be done at this time point.

### Actual behavior
When all data are compacted to most bottom level, the manual compaction is continue running for compacting the most bottom level again.

### Steps to reproduce the behavior
Run a manual compaction on full range on a db which has at least 2 levels.

We find this issue by continually dump the stat "rocksdb.sstables" and check "being_compacted" flag for each sst file.