Fix benchmark compile warnings and errors

[TheBlueMatt]

Hopefully soon once a few more PRs get merged we can require this
in CI so we won't have any more regressions here.

[codecov]

Codecov Report

Merging #875 (daa1979) into main (8088e4b) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #875      +/-   ##
==========================================
+ Coverage   90.20%   90.22%   +0.02%     
==========================================
  Files          55       55              
  Lines       28913    28994      +81     
==========================================
+ Hits        26080    26160      +80     
- Misses       2833     2834       +1     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	83.04% <ø> (ø)
lightning/src/routing/router.rs	95.79% <ø> (ø)
background-processor/src/lib.rs	95.77% <0.00%> (-0.23%)	⬇️
lightning/src/ln/functional_tests.rs	96.86% <0.00%> (+0.01%)	⬆️
lightning-persister/src/lib.rs	96.20% <0.00%> (+1.24%)	⬆️

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[valentinewallace]

Should we fix this warning in benchmark: https://i.imgur.com/Zm4JLwj.png while we're at it? (Disregard if we're choosing not to fix it for whatever reason)

[TheBlueMatt]

Should we fix this warning in benchmark: https://i.imgur.com/Zm4JLwj.png while we're at it? (Disregard if we're choosing not to fix it for whatever reason)

Sure, why not.