Fix the throttle oversupply counting

[mhutchinson]

The previous implementation had false positives for oversupply when the
limit was increased at runtime due to the channel not being drained than
the for loop could put 2N tokens in N pigeonholes.

The new implementation uses a timeout for each episode of the throttle
that will terminate the for loop early if the tokens can't all be sent
down the channel. I considered using the next tick of the main ticker as
the timeout, but this would have caused the next episode of the throttle
to be skipped without more complexity being added.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 29.23%. Comparing base (07aa77d) to head (66cfaf9).
Report is 38 commits behind head on main.

Files	Patch %	Lines
hammer/hammer.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #124       +/-   ##
===========================================
- Coverage   55.99%   29.23%   -26.77%     
===========================================
  Files           8       15        +7     
  Lines         759     1406      +647     
===========================================
- Hits          425      411       -14     
- Misses        265      916      +651     
- Partials       69       79       +10     

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