Expose the cas_id in binary protocol.

[letmutx]

This will be useful for letmutx/macgyver-lock-rs#10

[letmutx]

One thought is that we could have a BinaryClient which works only with binary protocol and have methods that would take cas_id for all operations as necessary. @aisk thoughts?

[aisk]

I think it's cool to have this method, and for ASCII protocol, seems we can using the mget related API to implement it?

https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L472

[aisk]

Sorry maybe I had some misunderstanding, what you want is a single function to get a cas_id from a key without the total value body? Or something else?

[letmutx]

Yes, metaget and metaset seem to be similar to the ASCII commands to the binary commands I was talking about.

Implementing the feature I want in the binary protocol is much less work with the current code we have, though.