server: Change verification randomness variable meaning

[thomshutt]

What does this pull request do? Explain your changes. (required)
Change the meaning of the "Verification Frequency" parameter from meaning the odds of it running on a given request are 1 - 1 / VerificationFreq to 1 / VerificationFreq

How did you test each of these updates (required)
Wrote unit tests for method that calulates the odds

Does this pull request close any open issues?
No

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[oscar-davids]

LGTM!

[codecov]

Codecov Report

Merging #2385 (db0eac3) into master (e3f9286) will increase coverage by 0.02104%.
The diff coverage is 94.11765%.

@@                 Coverage Diff                 @@
##              master       #2385         +/-   ##
===================================================
+ Coverage   54.48659%   54.50763%   +0.02103%     
===================================================
  Files             93          93                 
  Lines          19458       19467          +9     
===================================================
+ Hits           10602       10611          +9     
  Misses          8267        8267                 
  Partials         589         589                 

Impacted Files	Coverage Δ
server/broadcast.go	78.22507% <94.11765%> (+0.18079%)	⬆️

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