Indexing: force refresh when versionMap is using too much RAM

If the user sets a high refresh interval, the versionMap can use
unbounded RAM.  I fixed LiveVersionMap to track its RAM used, and
trigger refresh if it's > 25% of IW's RAM buffer.  (We could add
another setting for this but we have so many settings already?).

I also fixed deletes to prune every index.gc_deletes/4 msec, and I
only save a delete tombstone if index.gc_deletes > 0.

I think we could expose the RAM used by versionMap somewhere
(Marvel?  _cat?), but we can do that separately ... I put a TODO.

Closes #6378
Why not prune deletes here? I don't think it matters much but since we clean almost every other thing here? (index writer, translog etc). 

Also - don't we need to clear to the current version map here? (same as in the COMMIT_TRANSLOG case)