Feature request: support for meta commands #22

Memcached has some new 'meta commands' that extend the text protocol, but do not appear to be supported in memcached-client (and presumably not in the njs server either. I'm not sure if the parser would need changes.)

There is an overview at https://github.com/memcached/memcached/wiki/MetaCommands

They are fully documented here:

• Meta Debug me
• Meta Get mg
• Meta Set ms
• Meta Delete md
• Meta Arithmetic ma
• Meta No-Op mn

I tried using cmd() with a meta command to retrieve the expiration time of a key that I had previously set:

client.cmd('mg mykey t')
  .then(console.log.bind(console, 'success'))
  .catch(console.error.bind(console, 'error'))

but it logs an error and then times out:

No command action defined for [ 'HD', 't31' ]
error Error: Command timeout
[stack trace]

(In the command, mg is for "meta get", t indicates that I want the expiration time. In the response, HD indicates that there is no value, only headers, and t31 indicates that there are 31 seconds left until the expiration.)

I also tried adding the v flag to request a value in addition to the expiration time. It sort-of worked, in that it returned the value rather than timing out. But it still complained rather than returning the expiration time:

No command action defined for [ 'VA', '1', 't60' ]
success 2

I particularly want the Meta Arithmetic command so that I can use it in the rate-limit-memcached rewrite that we're working on to do an "upsert" increment and retrieve the value and TTL all in a single command.

I'd be willing to do some of the work to add these new commands, but I thought it would be smart to start a discussion here to plan out the overall shape of things.