Fix a race in decrypting megolm messages (#544)
* Fix a race in decrypting megolm messages
This fixes a race wherein it was possible for us to fail to decrypt a message,
if the keys arrived immediately after our attempt to decrypt it. In that case,
a retry *should* have been scheduled, but was not.
Fixes https://github.com/vector-im/riot-web/issues/5001.
* WORDS
