Some historic messages are undecryptable, but are loaded much later

Sometimes, often when you scroll a long way up your timeline, you encounter occasional unable-to-decrypt messages. (Empirically, it seems like the further back you go, the more UTD messages you find.)

Often, if you come back later on, some of those UTD messages are decrypted.

When we look at the logs, we see:
* Decryption error
* Key retrieved from backup
* But no attempt to re-decrypt (until much later)

In other words, this sounds much the same as #3768, but it's intermittent.

Start by comparing this intermittent behavior with issue #3768 and tracing the logs from the decryption error through key retrieval from backup. Done means historic unable-to-decrypt messages are retried promptly after their keys are retrieved, rather than remaining undecryptable until much later.

Written by the indexing model from the issue text.