ref(crons): Simplify monitor clock implementation

[evanpurkhiser]

This is a follow up to GH-54204 as suggested by @fpacifici

#53661 (comment)

Can we just have the pulse message running in both modes and treat
everything as high volume mode?

Instead of having two modes, we can simply always use the same logic for
dispatching the monitor tasks on the minute roll-over, using the
consumer as a clock.

Previously the worry here was that in low-volume check-in situations
nothing would drive the clock and we would need to have an external
clock, with a different way to dispatch the tasks. But there is no need
for a different way to dispatch the tasks, we can have an external clock
that pulses messages into the topic and we can simply use the same logic
already implemented to use the topic messages as a clock.

This change removes the concept of "high volume" / "low volume" and adds
the concept of a "clock_pulse" message to the consumer.

In a follow up PR we will introduce the celery beat task which produces
the clock_pulse messages.

[codecov]

Codecov Report

Merging #54388 (a2b55a6) into master (6612cd8) will increase coverage by 0.00%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #54388   +/-   ##
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  Coverage   79.74%   79.74%           
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  Files        4991     4991           
  Lines      211939   211944    +5     
  Branches    36115    36117    +2     
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+ Hits       169008   169018   +10     
+ Misses      37731    37728    -3     
+ Partials     5200     5198    -2     

Files Changed	Coverage Δ
src/sentry/monitors/consumers/monitor_consumer.py	89.40% <100.00%> (+1.09%)	⬆️

... and 3 files with indirect coverage changes