lnpeer: async `maybe_send_commitment`

[SomberNight]

this builds on top of https://github.com/spesmilo/electrum/pull/7835 (first commit is from there)

This is re https://github.com/spesmilo/electrum/commit/cc1b4a5c903ac82d20d8107f45d829d8d1270a80#commitcomment-74922012

• Await pong before sending commitment_signed (per BOLT-2)

We await self.ping_if_required() inside maybe_send_commitment(). This needed making maybe_send_commitment async, which really quite snowballed o.O

Anyway, this is a separate PR from https://github.com/spesmilo/electrum/pull/7835, as that PR is useful in itself.

[SomberNight]

I've changed this to draft as it needs some discussion before a potential merge.