Feature/add gomemcache

Add optional golang based gomemcache library. 
This library does not require libmemcached, and provides it's own pooling, but does not offer the level of control that gomc does. This library has NOT yet been fully load tested.

@bbangert || @kitcambridge r?
Let's move this to the top of the function, so that we're not reading from memcached and allocating a `ChannelRecord` for invalid keys. I'll make the same change to `emcee_store` adapter.