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.
```
      // todo: the baseScanner may have more increment-by-delta cells that have to be merged into one counter value,
      //       which we don't verify by only looking at the "addedCnt < limit" condition. Hence, if the limit set on scan is less
      //       than number of increment-by-delta cells for a counter, it may result in multiple values of the same cell
      //       being returned or a partial summation result to be returned. It may not be a problem as we don't yet use
      //       limit-on-scan explicitly for table datasets. But it will cause issues when we do.
      //       See CDAP-971.
```