SIFS Removed keys do not count towards free size

SoftIndexFileStore keeps track of a files size by when a value is no longer referenced. Unfortunately, a tombstone always counts towards the file data size, when it should only count towards the file data size when there are other references to that key.

Thus if you have a pattern of deleting a lot of keys that fill up a data file that file will never compact if the amount of data removed is greater than the compaction ratio.