memcached plugin: Refactor the memcached_query_daemon() function.

memcached plugin: Refactor the memcached_query_daemon() function.

The connecting code has been broken out in separate functions and the
writing and reading from the socket no longer uses poll(2),
non-blocking I/O and a custom built retry logic. Instead block on I/O and
let the read-thread-pool do its thing.