We should prioritise established olm sessions over half-open ones when sending outgoing messages

[richvdh]

For example:

• Alice and Bob want to talk to each other at the same time
• They both create a new Olm session with the other and send a pre-key message down it:
  • Alice creates Olm session A and sends Bob a message down it
  • Bob creates Olm session B and sends Alice a message down it
• Now, suppose:
  • Alice fails to decrypt the pre-key message on session B from Bob (for example, due to #2782)
  • However, Bob successfully decrypts the pre-key message on session A from Alice
• When Bob next wants to contact Alice, he should prioritise session A, rather than session B.

[lampholder]

/me throws a handful of labels at this