Add stats.countby Storm command (SYN-6183)

[Cisphyx]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9a283f3) 97.31% compared to head (5094620) 97.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3385      +/-   ##
==========================================
- Coverage   97.31%   97.22%   -0.09%     
==========================================
  Files         230      231       +1     
  Lines       46896    46966      +70     
==========================================
+ Hits        45637    45663      +26     
- Misses       1259     1303      +44     

Flag	Coverage Δ
linux	97.22% <100.00%> (+<0.01%)	⬆️
linux_replay	?

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Files	Coverage Δ
synapse/cortex.py	97.01% <100.00%> (-0.49%)	⬇️
synapse/lib/storm.py	95.71% <100.00%> (ø)
synapse/lib/stormlib/stats.py	100.00% <100.00%> (ø)
synapse/lib/stormtypes.py	98.80% <ø> (+0.03%)	⬆️

... and 8 files with indirect coverage changes

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