Refactor \OC\Memcache\Factory

Caches divided up into two groups: distributed and local. 'Low latency' is an
alias for local caches, while the standard `create()` call tries to get
distributed caches first, then local caches.

Memcache backend is set in `config.php`, with the keys `memcache.local` and
`memcache.distributed`. If not set, `memcache.distributed` defaults to the value
of `memcache.local`.