"metaget" support (mget, mset, mdelete)

Yo! I see xmemcached looks like the most actively developed java client I can find.

memcached/memcached#484 - I've been developing on this for a while, and after a break intend to finish it up. I'm personally happy with the general approach and the code is at the 90% mark. What I'd love to hear now is some feedback from client authors/maintainers on how good/bad/annoying this would be to implement.

I can clear up/rewrite text/add examples if it's not clear enough right now. The protocol is low level, and I'd expect a raw-ish interface from clients. Most users will only be needing a couple of high level patterns, which should be easy to implement wrapping the raw protocol (a herd-protected get, stale-while-revalidate, etc). I haven't written these examples up explicitly yet while I look for feedback.

Thanks!

No repository file, test, or entry point is named. Start by reading the xmemcached project context and the referenced memcached/memcached#484 protocol work; identify the client surface and tests that would need to cover mget, mset, and mdelete before defining what complete support means.

Written by the indexing model from the issue text.