Store state when sending commitment_signed

[t-bast]

If we only store state when sending tx_signatures, there are cases where we cannot reconcile states if a disconnection occurs during the signing steps: one side will have sent tx_signatures and thus must wait for the transaction to be spent or double-spent, while the other side has already forgotten that channel because they haven't sent tx_signatures.

This is fixed by storing state when sending commitment_signed, and adding a next_funding_txid field to channel_reestablish to ask our peer to retransmit signatures that we haven't received.

[t-bast]

@niftynei @dunxen @morehouse this commit adds the requirements addressing in this comment. Let me know if that's clear enough, and any feedback you have from implementing it or going through the message flow scenarios described in my comment or in https://gist.github.com/t-bast/1ac31f4e27734a10c5b9847d06db8d86

[dunxen]

Thanks for this and for enumerating the disconnection scenarios in the gist! It seems clear IMO, but I will bring it up in LDK's project sync this week and see if there's anything glaringly tricky here. :)

[t-bast]

I slightly updated the requirements: the receiver of channel_reestablish doesn't need to retransmit its commit_sig if it has already received the remote tx_signatures (because since the sender has sent tx_signatures, they have stored that commit_sig already), as pointed out by @niftynei

[dunxen]

Thanks, LGTM!

[morehouse]

LGTM

[t-bast]

Squashed the fixup commit in 248caeb

[niftynei]

ACK 248caeb