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.
I still disagree with you. The user have no idea what the deferred does, maybe the deferred don't have any callback attached and will be fired with defer.success. The user don't need to know the actual implementation, she just needs to know that their callbacks will be fired when the deletion is completed. AFAIK twisted people document from that point of view.

I don't like over documented stuff, documentation should be short and precise. Your solutions are way too long saying stuff that you don't need to know. You just care about when will your callbacks be fired and what is the result value of the deferred.