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.

src/crypto/algorithms/base.js

@@ -117,7 +117,8 @@ class DecryptionAlgorithm {
      *
      * @param {MatrixEvent} event undecrypted event
      *
-     * @return {Promise} resolves once we have finished decrypting. Rejects with an
+     * @return {Promise<module:crypto~EventDecryptionResult>} promise which
+     * resolves once we have finished decrypting. Rejects with an
      * `algorithms.DecryptionError` if there is a problem decrypting the event.
      */