Applying patch so that UDP requests cannot cause the server to spin out of control

Patch from Upstream Memcached - https://github.com/memcached/memcached/commit/954f6dddc41c80d2e625bce63744df5556a425bb

Don't permanently close UDP listeners on error
Also, don't inflate rbytes as we can only ever process one UDP packet at a
time.

Patch by pi3orama. Fixed by Dormando to use the correct state and actually
work.

I'm not fully convinced of the rbytes bit. We must ensure that a UDP packet
with multiple commands in it has all of its commands processed before we can
possibly call recvfrom again.
Merge pull request #21 from cloudflare/master

Twemcache starts spinning with 100% CPU because of bad UDP packages