DynComms [1/n]: Implement Quiescence Protocol

[ProofOfKeags]

Rebased on #9097 now.

NOTE: This PR is part of a series implementing Dynamic Commitments. This PR does not directly implement any Dynamic Commitments specific logic but quiescence is a protocol gadget that is a prerequisite for Dynamic Commitments.

Change Description

This change implements the behavior described in the Quiescence Specification. It allows us to respond to our peer's request to quiesce the channel as well as implementing some ChannelUpdateHandler operations that allow us to initiate the process ourselves.

Some commits towards the end of the series have been included to allow us to initiate quiescence via RPC for the purposes of integration and interop testing. These commits should be removed before this PR is considered ready to merge. They will ultimately be replaced by RPCs that initiate the Dynamic Commitments protocol itself which will implicitly initiate quiescence as part of its process.

NOTE: This PR does NOT include a mechanism for timing out a quiescence session. This means that if we have an intentionally or unintentionally uncooperative peer, the channel will remain quiesced indefinitely. This is not desirable and will either be addressed in later commits in this PR or into a subsequent PR. However, this PR is submitted without it as it is "complete" in its own right.

Steps to Test

Steps for reviewers to follow to test the change.

Pull Request Checklist

Testing

• [ ] Your PR passes all CI checks.
• [x] Tests covering the positive and negative (error paths) are included.
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Walkthrough

The implementation of the Quiescence (stfu) protocol introduces a new mechanism for initiating quiescence on a link via RPC, enhancing channel synchronization, and error handling. It includes a state machine for managing the quiescence protocol, new RPC methods for testing purposes, and extends the functionality of existing structures to support these features. This change is pivotal for the advancement of Dynamic Commitments within the Lightning Network.

Changes

File Pattern	Change Summary
htlcswitch/...	Introduced InitStfu() method, quiescence protocol logic, and related error handling.
itest/...	Added test cases for validating the quiescence protocol.
lnrpc/...	New RPC method Quiesce and related entities for quiescence protocol handling.
lnwire/...	Added MsgStfu message type and quiescence-related feature bits.
peer/...	Handling of lnwire.Stfu messages.

Assessment against linked issues

Objective	Addressed	Explanation
Implement Quiescence (stfu) as a prerequisite for Dynamic Commitments (#8260)	✅
Track state of Dynamic Commitments and upgrade channels to Taproot Channels (#7878)	❌	The PR focuses on the Quiescence protocol, not directly on Dynamic Commitments or Taproot Channels.
Address retransmission of shutdown message upon reconnection (#8397)	❌	This PR does not address the retransmission issues described.
Improve handling of dust HTLCs in channel closure (#7969)	❌	The changes are unrelated to dust HTLCs handling.

Possibly related issues

• #7878: While this PR doesn't directly implement Dynamic Commitments, the Quiescence protocol is a foundational step toward that goal.
• #8397: Though not directly addressed, improvements in channel synchronization and error handling could indirectly benefit the retransmission logic in future updates.

🐇✨
In the land of code and wire,
A rabbit hopped, with dreams so dire.
"Let's quiesce," it said, with glee,
For quieter channels, we all agree.
With stfu in place, and tests to run,
Our Lightning paths are second to none.
🌩️🐰💻

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[lightninglabs-deploy]

@roasbeef: review reminder @proofofkeags, remember to re-request review from reviewers when ready

[ProofOfKeags]

Haven't reviewed the code, just wanna say maybe we could update the ruleset here so you can push branches and change the base branch so it's easier to review.

I'm not sure I understand what's being proposed here? Is this just so that I can have multi-stage branching in the LND main repo itself? If so, what does the ruleset do to support this?

[lightninglabs-deploy]

@roasbeef: review reminder @proofofkeags, remember to re-request review from reviewers when ready

[carlaKC]

While we're in the business of pulling things out into prefactors, how about pulling out 3634cbafbc1d191a44ac4df2315cddad432ace0e and 8ccdbfd491f700c427376ba9df0bbfb33c6d2af9 into a trivial PR as suggested by @yyforyongyu?

[ProofOfKeags]

While we're in the business of pulling things out into prefactors, how about pulling out https://github.com/lightningnetwork/lnd/commit/3634cbafbc1d191a44ac4df2315cddad432ace0e and https://github.com/lightningnetwork/lnd/commit/8ccdbfd491f700c427376ba9df0bbfb33c6d2af9 into a trivial PR as suggested by @yyforyongyu?

#8962 is served. 👨🏻‍🍳🔪🥘

[ProofOfKeags]

All comments addressed. The code is substantially different. I recommend a full review. Only a handful of comments were contested as opposed to accepted. Feel free to consult those threads for the objection raised.