Add `getMulti` to `mcrouter` extension

**Is your feature request related to a problem? Please describe.**
`memcached` (and `mcrouter` since it also implements the `memcached` spec) allows multiple keys to be provided to the `get` and `gets` commands which in turn will return multiple results. ([Protocol Reference](https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L296))

However, the API offered by the HHVM Extension for McRouter only has a single-key `get` method implemented. ([HHVM Docs](https://docs.hhvm.com/hack/reference/class/MCRouter/get/))

However, the `memcached` extension included in HHVM _does_ include a `getMulti` which offers this functionality. ([phpdoc reference](https://www.php.net/manual/en/memcached.getmulti.php))

**Describe the solution you'd like**
It'd be great if the `mcrouter` HHVM extension also offered a binding for `getMulti` to bring parity with the `memcached` extension.

**Describe alternatives you've considered**
Two main alternatives

1. Keep using `mcrouter` but just only with single-key fetches
This is a fine workaround and what I've been doing for now. But just thinking longer-term, it's not exactly ideal to have to make a single request per key fetched from memcached when potentially dozens of objects might be fetched (e.g. to load a feed).

2. Switch to the `memcached` extension instead of `mcrouter`
This is what I will probably wind up doing if I eventually need this functionality. `mcrouter` is nice but I'm definitely not working at such a scale where I need its functionality more than vanilla `memcached`'s

**Additional context**
I'd be happy to work on this in spare time. I'm sure the core team has more important things to tackle. I mostly want to make sure there isn't anything in the pipeline that'll jeopardize the value of embarking on this work on my own e.g. if the `mcrouter` extension were gonna be wholesale replaced or something.

Start by locating the mcrouter extension's existing `get` binding and compare it with the `memcached` extension's `getMulti` API and the referenced memcached protocol behavior. Done means the mcrouter HHVM extension exposes equivalent multi-key fetching and returns multiple results consistently with the existing memcached extension.

Written by the indexing model from the issue text.