signer: Refresh on nonces >= 500

[j0sh]

The orchestrator caps nonces at 600 so require a refresh at 500 in case the payment generates 100 tickets (the maximum).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.58179%. Comparing base (90ff849) to head (9629ce8).

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #4028         +/-   ##
===================================================
+ Coverage   35.54150%   35.58179%   +0.04029%     
===================================================
  Files            174         174                 
  Lines          45226       45231          +5     
===================================================
+ Hits           16074       16094         +20     
+ Misses         27879       27863         -16     
- Partials        1273        1274          +1     

Files with missing lines	Coverage Δ
server/remote_signer.go	67.40891% <100.00000%> (+0.33325%)	⬆️

... and 5 files with indirect coverage changes

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Files with missing lines	Coverage Δ
server/remote_signer.go	67.40891% <100.00000%> (+0.33325%)	⬆️

... and 5 files with indirect coverage changes

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