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, bradfitz/gomemcache#156 and pinterest/pymemcache#272. There is also a python memcached client with meta support https://github.com/RevenueCat/meta-memcache-py, and a go memcache client with meta support https://github.com/QuangTung97/go-memcache

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

[jonathon-geeves]

This would be of interest to us - quite a number of the features available in the new meta protocol are features which I have had experiencing building workarounds for via adding metadata into the payload stored against key values. However the direct support in the protocol would make the implementation more efficient, and allow simplification of the code.

A example of one of these is the Early Recache functionality. I've seen a workaround where clients store a refresh time in the value, and then perform a fuzzy comparison on the client side to determine who should re-cache the value (so comparing recache time against current time, with some randomised offset), and then the client must write an update to the value to indicate that it is in the process of refreshing it.