Backport supress stateless packet to 0.10.x

[lijunwangs]

Backport e652b6d from main to 0.10.x branch.

The implementation is different from the main branch due to the refactoring happened to main branch.

1. The enforcement of the limit is done at the quinn-proto layer as it is where the endpoint generated packets being queued.
2. The transmit_queue_contents_len to track the buffer contents length at quinn-proto::Endpoint layer.
3. The outgoing_queue_contents_len to track the buffer contents length at Quinn::Endpoint layer
4. Quinn::Endpoint uses the new interface set_socket_buffer_fill exposed by proto::Endpoint to communicate the updated outgoing_queue_contents_len

[lijunwangs]

Hi Ben/Dirkjan, can you please review the backport implementation of e652b6d to the 0.10.x release? @Ralith @djc

[Ralith]

Ah, I see why you were thinking about separate methods now. I'm happy either way.

[Ralith]

This looks good so far, but did you also want to disable version negotiation and stateless reset packets under the heavily loaded condition?

[lijunwangs]

This looks good so far, but did you also want to disable version negotiation and stateless reset packets under the heavily loaded condition?

Great point! I reviewed the implementation in main -- we were suppressing version negotiation and stateless reset packets as well. I will add these into the consideration as well to keep it consistent and more robust.

[Ralith]

LGTM, thanks!