Restart broken Olm sessions
* Start a new Olm sessions with a device when we get an undecryptable
message on it.
* Send a dummy message on that sessions such that the other end knows
about it.
* Re-send any outstanding keyshare requests for that device.
Also includes a unit test for megolm that isn't very related but came
out as a result anyway.
Includes https://github.com/matrix-org/matrix-js-sdk/pull/776
Fixes https://github.com/vector-im/riot-web/issues/3822
