raft: Plug pseudo-random number generator

[freeekanayaka]

This plugs the internal PRNG into struct raft. For now the generator is seeded using struct raft_io->random(), but eventually it will be consumer's responsibility to initialize it.

[codecov]

Codecov Report

Patch coverage: 93.93% and project coverage change: -0.01% ⚠️

Comparison is base (330db74) 75.22% compared to head (ccd6415) 75.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   75.22%   75.22%   -0.01%     
==========================================
  Files          51       51              
  Lines        8964     8992      +28     
  Branches     2215     2220       +5     
==========================================
+ Hits         6743     6764      +21     
- Misses        987      992       +5     
- Partials     1234     1236       +2     

Files Changed	Coverage Δ
src/fixture.c	91.02% <92.59%> (+0.01%)	⬆️
src/election.c	78.91% <100.00%> (ø)
src/raft.c	77.24% <100.00%> (+0.64%)	⬆️

... and 1 file with indirect coverage changes

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