Drop invalid PDUs instead of erroring

[matrixbot]

This issue has been migrated from #7543.

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This is based on a conversation at matrix-org/matrix-spec-proposals#2540 (comment), summarized below:

Currently when an incoming federation event is "bad" for some reason it is rejected by returning a 400 error. This is particularly troublesome in endpoints where multiple events are handled at once, as the entire transaction gets rejected.

Reasons an event might be rejected include:

• events with no type or depth
• events with bad depth values
• events in v{1,2} rooms with no event_id/v3+ rooms with an event_id
• events in v6 rooms that have integers out of range or floats
• events with too many prev_events (Dummy issue #3124)
• etc.

There are three proposed options for this situation:

1. silently drop the event. I would argue that we should not be blackholing events, ever: it feels like we'll end up dropping events unexpectedly.
2. send an error back in the response, against the event's event_id. This, of course, requires the recipient to parse the json, remove a couple of properties, then re-encode it using canonicaljson (which, you recall, is theoretically impossible) to calculate the event id. That sounds like a tautology to me. It also requires servers which don't even support older room versions to still accept transactions with floats in them. So that leaves us with:
3. reject the whole lot.

It is potentially difficult to return a sensible error since (theoretically) you might not even be able to parse the event data and thus it is proposed to silently drop these events for now.