Refactor `on_receive_pdu` code (#10615)

* drop room pdu linearizer sooner

No point holding onto it while we recheck the db

* move out `missing_prevs` calculation

we're going to need `missing_prevs` whatever we do, so we may as well calculate
it eagerly and just update it if it gets outdated.

* Add another `if missing_prevs` condition

this should be a no-op, since all the code inside the block already checks `if
missing_prevs`

* reorder if conditions

This shouldn't change the logic at all.

* Push down `min_depth` read

No point reading it from the database unless we're going to use it.

* Collect the sent_to_us_directly code together

Move the remaining `sent_to_us_directly` code inside the `if
sent_to_us_directly` block.

* Properly separate the `not sent_to_us_directly` branch

Since the only way this second block is now reachable is if we
*didn't* go into the `sent_to_us_directly` branch, we can replace it with a
simple `else`.

* changelog