Add support for meta commands

[QuChen88]

Meta Commands protocol is a powerful replacement of the traditional Memcached commands including GET/SET/INCR/DECR/DELETE/CAS with support for advanced caching use cases. It was a top highlight of memcached 1.6, see https://github.com/memcached/memcached/wiki/ReleaseNotes160. However, there appears to be a lack of client support for the meta command protocol, preventing adoption for this new capability.

Currently there is interest in supporting meta command in several other memcached clients, see electrode-io/memcache#22 and pinterest/pymemcache#272. There is also a python memcached client with meta support https://github.com/RevenueCat/meta-memcache-py.

This is the tracking ticket for implementing the new Meta Commands protocol. See https://docs.memcached.org/protocols/meta/