original `send` method is blocking, and it is used as such by Wokkel and thus can't be
changed to an async method easily. However, an Async method is necessary to have an async
trigger at the very end of the send workflow for end-to-end encryption. To workaround
that, `send` is an async method which call `a_send`, an async method which actually does
the sending. This way legacy method can still call `send` while `a_send` can be await
otherwise.
Fix calls to `sendMessageData`: the method now being an `async` one, `ensureDeferred` had
to be used in some calls.
ee5784811e