* OlmDevice.hasInboundSessionKeys
* OlmDevice.getInboundGroupSessionKey
The latter means that MegolmDecryption.shareKeysWithDevice takes longer before
it sends out the keyshare, so means the unit test needed an update
... instead of having it call event.setClearData.
The main advantage of this is that it fixes a race condition, wherein apps
could see `event.isDecrypting()` to be true, but in fact the event had been
decrypted (and there was no `Event.decrypted` event on its way).
We're also fixing another race, wherein if the first attempt to decrypt failed,
a call to `attemptDecryption` would race against the first call and a second
attempt to decrypt would never happen.
This also gives a cleaner interface to MatrixEvent, at the expense of making
the `megolm` unit test a bit more hoop-jumpy.
when we receive a m.forwarded_room_key, add it to the crypto store, but
remember who forwarded it to us, so we can decide whether to trust them
separately.
