reimplement db:reindex_search using tire api; avoid OOM errors

@kevinchugh @gwprice 

Herein lay (squashed) the changes needed to fix the reindex job.  This version is running on loadtest right now; there's some chance it may fail revealing some new issue, but it may be many hours before that happens.

The key elements of the fix were:
 * use tire api instead of rake, and leverage its pagination capability
 * ensure that memory mapping and unit of work features are really disabled
 * force periodic GC (the way it is done in Tire >= 0.6.0, while we are still using an older version)