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.

You'll find install instructions in documentation (see doc/installation.rst).

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