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
That makes me nervous, because then all pruning of the tombstones is driven by wall clock time ... and I don't like relying on that (e.g., what if the clock shifts and suddenly we don't prune for a very long time...).  I'll add a comment explaining this reasoning ...