when a peer jid was not in our roster, devices list was not retrieved, resulting in failed
en/decryption. This patch does check it and download missing devices list in necessary.
There is no subscription managed yet, so the list won't be updated in case of new devices,
this should be addressed at some point.
OLDMEMO was applied at the `send_message_data` trigger level, however this was missing
stanza constructed without message data (which is notably the case for XEP-0308). This has
been fixed by using the `send` trigger instead. As a side effect, the same code is now
used for TWOMEMO and OLDMEMO, making it simpler.
