port to new soledad async api

**Not ready to merge, missing changes in leap.mail and maybe other to be compatible with the new API**

It needs to synchronize the merge with the soledad async API and the upgrades in leap.mail.
This will be the last one to coment in regard of Deferred docstrings, I promise. ;-)

I'm being picky, but the Deferred itself doesn't have a list of keys. Maybe we should think about a way to standardize the doc along all LEAP async modules so we avoid this discussion in the future. As I see it, the most accurate way would be to say: "A Deferred whose callback will be handed a list of all keys in local db."

Not sure right now how would be a good way to standardize it.