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.
Related to the question I raised above about the use of `defer.succeed()`, what would be the difference of doing `dpub = defer.Deferred()` and `dpriv = defer.Deferred()` here?