Bug #64144: Process failover requests

See the comments at https://bugs.php.net/bug.php?id=64144 for an explanation of why this is necessary. Failover get requests are added to a pool's pending queue, but then are not run until the next memcache API call, by which time the location into which the return value is written is no longer valid. I'm not sure how/if failover get requests have ever worked.

However, I am far from 100% certain that this patch is right, since I do not have a complete understanding of the code.