Messages can decrypt successfully on a retry but still show as UTD

[dbkr]

A number of rageshakes seem to be indicating that we're decrypting messages on a retry but they're still showing as undecryptable, according to the user.

Only pattern is that the message arrives before the key, the event fails to decrypt, a key request gets queued, the key gets decrypted, a decryption retry gets queued for the event, and then the key request gets cancelled, implying that the event decrypted successfully.

May perhaps be more common during a large catchup sync when the client will have a big sync result to process with a number of toDevice messages with keys.

[jryans]

Assuming @dbkr is investigating this.

[dbkr]

Don't think I've seen this since