PeerManager: add separate queue for gossip broadcasts

[valentinewallace]

This allows us to better prioritize channel messages over gossip broadcasts and lays groundwork for rate limiting onion messages more simply, since they won't be competing with gossip broadcasts in the channel message queue.

Also fixes a bug that was introduced in #1660, oops.

• docs

Lays groundwork for #1604

[TheBlueMatt]

Nice, thanks.

[TheBlueMatt]

LGTM. Can we now drop the enqueue_encoded_message utility now and only use the gossip variant?

[codecov-commenter]

Codecov Report

Merging #1683 (47e818f) into main (414d8e1) will decrease coverage by 0.05%.
The diff coverage is 46.42%.

@@            Coverage Diff             @@
##             main    #1683      +/-   ##
==========================================
- Coverage   90.91%   90.85%   -0.06%     
==========================================
  Files          85       85              
  Lines       45753    45763      +10     
  Branches    45753    45763      +10     
==========================================
- Hits        41596    41580      -16     
- Misses       4157     4183      +26     

Impacted Files	Coverage Δ
lightning/src/ln/peer_handler.rs	56.77% <46.42%> (-0.06%)	⬇️
lightning/src/chain/onchaintx.rs	94.02% <0.00%> (-0.92%)	⬇️
lightning/src/ln/payment_tests.rs	98.57% <0.00%> (-0.32%)	⬇️
lightning/src/ln/functional_tests.rs	96.84% <0.00%> (-0.20%)	⬇️
lightning/src/ln/monitor_tests.rs	99.35% <0.00%> (-0.13%)	⬇️
lightning/src/ln/channelmanager.rs	85.13% <0.00%> (-0.03%)	⬇️

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[jkczyz]

Looks good but should make commits atomic as per comment.

[TheBlueMatt]

If you're referring to another PR in the git commit, please refer to the specific commit by commit hash, not to github - git history should stand on its own without context from github.