core (xmpp): make `send` a blocking method, fix `sendMessageData` calls: 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.

Those certificates are used to activate TLS for end-2-end testing (to be as
close as possible as production environment), they are used in other containers
needing TLS certificates (notably Prosody).

To generate them, minica has been used. Minica can be found at https://github.com/jsha/minica.

The following commands have been used:

$ minica --domains "server1.test,*.server1.test,server2.test,server3.test,libervia-backend.test,libervia-web.test"
$ chmod 0644 minica.pem server1.test/cert.pem && chmod 0640 server1.test/key.pem

Note that certificates are valid for 2 years and 30 days, so they must be renewed after this delay.