Mutability does not seem to be required, and this guarantees we can
reuse the same Link instance for receiving and acking packets in turn.

When processing pending packets for the `clear packets` command,
schedule RecvPacket in both directions before proceeding with
AckPacket, also in both directions. This ensures we process also the
acks created by the packets just received.

There is no longer a "source" chain, but we need to clarify that both
directions of the channel are cleared even though only one end is used
for identification.

Use a span and KV pairs to provide the context in
ChainEndpoint::build_packet_proofs, so the log event macro
invocations are short and to the point.

This is the slowest implementation, not optimizing on any shared state
or eliminating redundant checks.

This is an option on Link::new_from_opts, so it seems logical to
also make it optional on the same level API.