Make crypto.decryptMessage return decryption results
... 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.
