Experiment with strategies for picking compaction candidates in LCS

Ideas taken here: http://rocksdb.org/blog/2921/compaction_pri/

Current strategy in LCS is that we keep track of the token that was last compacted and then we start a compaction with the sstable containing the next token (kOldestSmallestSeqFirst in the blog post above)

The rocksdb blog post above introduces a few ideas how this could be improved:

• pick the 'coldest' sstable (sstable with the oldest max timestamp) - we want to keep the hot data (recently updated) in the lower levels to avoid write amplification
• pick the sstable with the highest tombstone ratio, we want to get tombstones to the top level as quickly as possible.