Reliably deliver gossip messages from our `ChannelMessageHandler`

[TheBlueMatt]

When our ChannelMessageHandler creates gossip broadcast MessageSendEvents, we generally want these to be reliably delivered to all our peers, even if there's not much buffer space available.

Here we do this by passing an extra flag to forward_broadcast_msg which indicates where the message came from, then ignoring the buffer-full criteria when the flag is set.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.89744% with 47 lines in your changes missing coverage. Please review.

Project coverage is 90.20%. Comparing base (4147de2) to head (116e369). Report is 40 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/peer_handler.rs	72.78%	44 Missing and 2 partials :warning:
lightning/src/util/test_utils.rs	90.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3142      +/-   ##
==========================================
+ Coverage   89.68%   90.20%   +0.52%     
==========================================
  Files         126      127       +1     
  Lines      103306   110733    +7427     
  Branches   103306   110733    +7427     
==========================================
+ Hits        92651    99889    +7238     
- Misses       7936     8205     +269     
+ Partials     2719     2639      -80     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[TheBlueMatt]

Pushed some additional comments/docs.

[TheBlueMatt]

Added a test (and rebased, cause this old branch doesn't even build anymore...thanks MSRV-breakages...).

[TheBlueMatt]

Rebased.

[TheBlueMatt]

Squashed.