Use futures to make parts of the memcached driver async

Instead of having a sync api exposing async results in a
synchronous manner we can provide a asynchronous api by
just using a simple executor and using the futures that it
returns to map into the tooz async result concept.

This moves the simplistic methods that are not using the
retry decorator to being actually async, in a future change
the retry decorated ones can be altered as well...

Change-Id: I39d86da24b3f559890df12095ebde837e2820c63
Merge "Use futures to make parts of the memcached driver async"