Add fallback test, refactor other tests

* Verify that fallback partitions are not indexed.
* Refactor some of the yz_rt functions.
* Update other tests to make use of refactoring.
Don't index fallback data

Currently Yokozuna indexes fallback data.  This shouldn't be done for
the following reasons:

* Queries are never run against fallback data.  This is by design as
  fallback data, by virtue, is always a subset of data for a given
  partition.  It is only written to when the primary is not available.
  If queries were to run against fallbacks than they would be
  neglecting the primary data set.

* KV is the authoratative source for data.  Yokozuna is merely an
  index for that data.  Let KV handle the fallback cases and index
  that missing data when hinted handoff occurs.

* Uses extra CPU/disk for no reason.