Multiple servers in one SqlBagOStuff

Added a multi-server feature to SqlBagOStuff.

Tests done:
* With both the "shard" and multi-server features enabled and
  $wgAllDBsAreLocalhost, tested getMulti, set, incr, decr, keys,
  garbageCollect, deleteAll. Tested wiki page views with
  $wgMainCacheType set to such a cache.
* Tested connection errors.
* In the plain CACHE_DB configuration, tested wiki page views.

Change-Id: Ifba0d276ef724cc9b33f687bc0adae4637054328
Merge "Multiple servers in one SqlBagOStuff"