Allow round-robin across multiple URLs

[AlCutter]

This PR allows a single hammer instance to talk to multiple target frontends.

This allows it to reason about total load to the target system, but also gives it a better chance of detecting any split views which somehow happen but remain "self-consistent" per frontend.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 27.88%. Comparing base (07aa77d) to head (748f7f1).
Report is 63 commits behind head on main.

❗ Current head 748f7f1 differs from pull request most recent head 0901bd7

Please upload reports for the commit 0901bd7 to get more accurate results.

Files	Patch %	Lines
hammer/hammer.go	0.00%	30 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #151       +/-   ##
===========================================
- Coverage   55.99%   27.88%   -28.12%     
===========================================
  Files           8       15        +7     
  Lines         759     1474      +715     
===========================================
- Hits          425      411       -14     
- Misses        265      984      +719     
- Partials       69       79       +10     

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