SoftIndexFileStore should support a way to remove excess entries

SoftIndexFileStore as of 13 has become the default file store implementation. One feature that has been asked about a few times is whether this store has an option like max-entries that the SingleFileStore had.

Unfortuantely implementing this is not as trivial as it was for SingleFileStore as the latter had an in memory map holding all keys so it was quite easy to keep track of them (with obvious drawbacks of using substantial JVM heap).  Also to note that the SingleFileStore did not allow for max entries to be configured when the store was segmented.

We would have to think of how we could do this with SoftIndexFileStore. We have no way to track recent access like SingleFileStore did. We could possibly utilize either the segments and limit entries per segment and/or piggy back on compaction to remove some entries if there are too many in general. Using the compactor would be problematic without updates or removals (ie. all creates).