Don't blacklist a server on a failed TCP read/write - only on UDP failure or TCP connect failure
If a get/gets failed to connect to some servers, and didn't receive a
success response from the others, return MEMCACHED_CONNECTION_FAILED
so the caller knows the result is not definitive.
Merge pull request #1 from Metaswitch/memcached_restart_fix

[Reviewer: Mike] Memcached restart fix