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 can change it, but that will come in another pull-req as it needs changes in leap.mail and other stuff. The error is already in the namespace as all the exceptions in keymanager are error.KeyNotFound, don't know what makes more sense but this was here when I arrived.