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's a good question; I didn't add accounting for the tombstones because there's nothing we can actually do about it, i.e. we can't flush/refresh to clear them since they only clear once their index.gc_deletes has expired.

But I agree it would be nice to track this e.g. for stats purposes.  I'll try to add it ... it's tricky because the same ID can be in current/old and tombstones at the same time.  I think I'll just double-count the RAM?  It's only double counted until a refresh happens ....