Add max replica constraint

[xinaxu]

fixes #315
fixes #209

Adds a flag to limit number of replicas per PieceCID.
Long term goal is to offer a more granular way to define number of replicas. For now, we'll use a global flag.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eedbb4d) 73.92% compared to head (9946703) 73.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
+ Coverage   73.92%   73.93%   +0.01%     
==========================================
  Files         149      149              
  Lines        9813     9829      +16     
==========================================
+ Hits         7254     7267      +13     
- Misses       1809     1811       +2     
- Partials      750      751       +1     

Files	Coverage Δ
cmd/run/dealpusher.go	47.05% <100.00%> (ø)
service/dealpusher/dealpusher.go	74.24% <100.00%> (+1.01%)	⬆️
util/testutil/testutils.go	88.39% <100.00%> (+0.10%)	⬆️

... and 2 files with indirect coverage changes

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