plugin XEP-0384: fix calls outside of main thread: `callFromThread` is used instead of `callLater` with a 0 delay, as the later is not made to be called outside of main thread (which was resulting in huge and random delays during calls). fix `OmemoStorage._callMainThread` which was returning wrong Deferred due a late lambda evaluation.

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)