Decrypt initial message after joining new DM

[codemonium]

Suppose that Alice logs in using Element. Before this change, when Bob would send a DM to Alice through Pantalaimon, Alice would not be able to decrypt Bob's initial message. Instead, she would see

Unable to decrypt: The sender's device has not sent us the keys for this message.

and Pantalaimon's olmsessions table would have no associated records. Any future messages would be visible however. On the other hand, when Alice (using Element) is the one to send the first DM to Bob, he can decrypt the initial message.

For Pantalaimon to execute /keys/claim, get_missing_sessions must return the invitee's device (and log Missing session for device). If Pantalaimon calls this method too soon, then self.device_store will not have the device. To populate self.device_store before Pantalaimon calls get_missing_sessions, it must execute /keys/query (and invoke _handle_key_query) earlier, during the /createRoom request. Pantalaimon does execute the /keys/query request during a sync after the server finishes creating the DM (and logs Adding new device to the device store for user), but only after checking unsuccessfully for the device in self.device_store.

After this change, Pantalaimon executes /keys/claim, there is one record in olmsessions, and Alice can decrypt Bob's initial message.

[chookity-pokk]

This is a few years late but @codemonium if you want to rebase this on master I'd like to merge it.