Refactor long functions

[engelsanchez]

This is a reminder to go back and refactor long functions in the Bitcask code for readability. A good refactor without functionality change is due. Readability is definitely key to spot the races that we are constantly fighting in Bitcask.

Some candidates:

• bitcask:merge1/4
• bitcask:merge_single_tombstone/5
• bitcask:inner_merge_write/6

etc, etc.