Fail RGS data processing early if there is a chain hash mismatch

[dunxen]

No point in doing any extra processing if we don't even have a match for the chain hash.

Resolves #2242.

[codecov-commenter]

Codecov Report

Patch coverage: 95.00% and project coverage change: -0.02 ⚠️

Comparison is base (4dce209) 90.79% compared to head (142fdca) 90.78%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2324      +/-   ##
==========================================
- Coverage   90.79%   90.78%   -0.02%     
==========================================
  Files         104      104              
  Lines       53033    53052      +19     
  Branches    53033    53052      +19     
==========================================
+ Hits        48153    48163      +10     
- Misses       4880     4889       +9     

Impacted Files	Coverage Δ
lightning-rapid-gossip-sync/src/processing.rs	92.72% <93.75%> (-1.10%)	⬇️
lightning-background-processor/src/lib.rs	82.63% <100.00%> (ø)
lightning/src/routing/gossip.rs	89.81% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

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