With level merge enabled, we expect there are no more than 10 sorted runs of blob files in both of last two levels. But 

- Since we force using level_compaction_dynamic_level_bytes, last level blob files won't be merged again, sorted run number in last level will increase infinitely.
- Some hot range may contain more sorted runs.

This patch implement range merge:

- Check sorted run number of compaction range after last level compaction
- Mark blob files to merge in next compaction if sorted run number > max_sorted_runs