Fix stale gossip retention Heisenbug when channel updates are slow.

[arik-so]

Debugs and fixes #1914.

[codecov-commenter]

Codecov Report

Base: 90.73% // Head: 90.71% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (16deda0) compared to base (ac6e0b3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1953      +/-   ##
==========================================
- Coverage   90.73%   90.71%   -0.02%     
==========================================
  Files          97       97              
  Lines       50548    50551       +3     
  Branches    50548    50551       +3     
==========================================
- Hits        45864    45857       -7     
- Misses       4684     4694      +10     

Impacted Files	Coverage Δ
lightning/src/routing/gossip.rs	92.14% <100.00%> (+0.01%)	⬆️
lightning/src/ln/functional_tests.rs	96.93% <0.00%> (-0.13%)	⬇️
lightning/src/ln/monitor_tests.rs	99.45% <0.00%> (-0.11%)	⬇️
lightning/src/chain/channelmonitor.rs	90.99% <0.00%> (-0.10%)	⬇️
lightning/src/chain/onchaintx.rs	95.39% <0.00%> (+0.20%)	⬆️

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