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
Doesn't this mean that real-time get fails after delete? (in the sense that it will not find anything in the version map and fall back to look in the index, where the doc is not yet deleted.