prime-uids

We pre-fetch the uids from soledad on mailbox initialization
properly implement deferreds in several commands

Passing along a deferred as an observer whose callback will be
called with the proper result.

Returning to thread in the appropiate points.
just let's remember that twisted APIs are not thread safe!

SoledadStore process_item also properly returned to thread.

Changed @deferred to @deferred_to_thread so it results less
confusing to read.

"know the territory". aha!