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.
If there are specific issues, perhaps list them, otherwise, "we had bugs" doesn't add much
```
// Additional test to verify that single readless increments work correctly with gets and scans
```