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.
OK, my point here was not about using or not `C{x}`, but about phrasing it so we explicitelly say we're using "a key bound to an address" to sign, instead of abusing language and saying wre are using "an address" to sign. But i agree that is also obsessive, so feel free to ignore that proposal. :-)