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.
Yes, I was not sure how to mark that, I keep the *raise* notation as I didn't know how to document it better. Do we have any notation for the kind of failures that a deferred can have?