compaction: sstable output size

[petermattis]

The compaction sstable output size is unimplemented. The ShouldStopBefore mechanism/heuristic used in both RocksDB and LevelDB needs to be implemented. This heuristic adjusts the output sstable size based on the number of bytes the sstable overlaps in the "grandparent" level (the level below the output sstable level). Care should be taken to ensure that this takes into account range deletions in addition to puts, merges and point deletions.