CCBC-489, CCBC-490: Get-With-Replica fixes

This commit fixes numerous issues with the lcb_rget3()/lcb_get_replica
API calls as outlined in the issues above. In summary, this commit:

- Ensures the H_getreplica handler in handlers.c properly invokes the
  customized callback (i.e. rget_callback in operations/get.c)
- Pre-validates the status of the replicas for rget3(), preventing
  memory leaks in the face of insufficient replicas for the API call.
- Adds tests for the common use cases of get-with-replica

Change-Id: I38c794a11bf6093c6fb523aeb299698d76387b91
Reviewed-on: http://review.couchbase.org/40083
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>