Local: add optimization for when localMissing = []

Previously, even if the missing array was empty, we'd still call
getMultiple([]) on the backend and it would trickle down to who knows how far
down the call stack before being short-circuited.
getMultiple: optimize for empty($keys)

The Memcache backend does *not* optimize the case of empty($keys) and still
makes a round-trip on the network. We might as well make *our* code better.
Merge local--optimize-all-found (#8) into master

Local: add optimization for when localMissing = []

Previously, even if the missing array was empty, we'd still call
`getMultiple([])` on the backend and it would trickle down to who knows
how far down the call stack before being short-circuited.

After investigation, it turns out the php memcache library doesn't
optimize out the `->get([])` special case and still makes a network round
trip. Because of this I've added a `if (empty) return []` to each
implementation of `getMultiple()` as well just in case the caller is
dumb.