refactor: reuse timers instead of time.After in loops

[gammazero]

Reusing a timer repeatedly results in less GC than time.After in each loop iteration.

Closes #122

[codecov]

Codecov Report

Attention: Patch coverage is 29.41176% with 12 lines in your changes missing coverage. Please review.

Project coverage is 60.34%. Comparing base (ceb514c) to head (ffcef93).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
mfs/repub.go	12.50%	7 Missing ⚠️
bootstrap/bootstrap.go	0.00%	5 Missing ⚠️

@@            Coverage Diff             @@
##             main     #687      +/-   ##
==========================================
+ Coverage   60.32%   60.34%   +0.01%     
==========================================
  Files         243      243              
  Lines       31004    31018      +14     
==========================================
+ Hits        18704    18717      +13     
- Misses      10635    10637       +2     
+ Partials     1665     1664       -1     

Files with missing lines	Coverage Δ
bitswap/network/ipfs_impl.go	75.17% <100.00%> (+0.26%)	⬆️
bootstrap/bootstrap.go	31.94% <0.00%> (-0.61%)	⬇️
mfs/repub.go	75.23% <12.50%> (-5.38%)	⬇️

... and 11 files with indirect coverage changes