* More robust consuming of MLS messages

This commit changes the behaviour of `sendAndConsumeMessage` and
`sendAndConsumeCommitBundle` to actually wait for those messages on the
client's websocket.

This should fix a lot of the flakiness of MLS tests that appeared after
the introduction of message queuing.

* Fix testAppMessageSomeReachable

When some backends are down, the new `sendAndConsume*` functions do not
work, because they expect a message to be received by all clients.

This commit changes tests with such a scenario to only post the message,
and not consume it.

* Add protocol field to MLS test state

This is necessary because new users in mixed MLS conversations don't get
join events, and we are waiting for such events before consuming MLS
messages.

* Add CHANGELOG entry