Feature/fix incs compact

Follow up on https://github.com/caskdata/cdap/pull/796: hence has its commits too.

There's a bug in logic of merging already merged increments during compaction and flushes. The summary of the change: we now always store increments as deltas, never as values. This means you cannot override the value with simple put.. We can add this ability later - when not in rush.
Factor out this code? Seems like three identical blocks.