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 searched about style and could not find a general recommendation for this, but I always use the name of the argument if it is optional:

    d = self._get_keys(ktype, address, sign, fetch_remote=fetch_remote)