Compact level files at user key boundary

[kezhuw]

We currently use solution described in google/leveldb#375 to fix google/leveldb#320 (or its equivalent google/leveldb#376). Months ago, I known and realized that google/leveldb#339 is a better fix comparing to google/leveldb#375 due to obedience of file size limitation.

google/leveldb#339 fixes google/leveldb#320 by selecting upper boundary files to selected input level files. Due to the truth that user key level overlapping exists in all level files, I think we may/can simplify google/leveldb#339 to "compacting level files at user key boundary" instead "compacting input level files and their user key upper boundary files".