Add benchmark utility

[gkinsman]

This PR adds a little benchmark utility that is mostly borrowed from mtbench, found here.

It's a useful tool for testing queue throughput, and the effectiveness of the new consume pipeline.

The output looks like this (run from my local machine):

[codecov]

Codecov Report

Merging #845 (5d6ebcc) into master (59a3c3d) will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
- Coverage   81.83%   81.59%   -0.25%     
==========================================
  Files         113      113              
  Lines        2885     2885              
==========================================
- Hits         2361     2354       -7     
- Misses        524      531       +7     

Flag	Coverage Δ
linux	81.59% <ø> (-0.25%)	⬇️
macos	51.79% <ø> (ø)
windows	51.83% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...aying/Messaging/Monitoring/NullOpMessageMonitor.cs	70.00% <0.00%> (-10.00%)	⬇️
...ools/MessageHandling/Dispatch/MessageDispatcher.cs	87.20% <0.00%> (-5.60%)	⬇️
...tSaying/AwsTools/MessageHandling/SqsQueueByName.cs	97.56% <0.00%> (+1.21%)	⬆️

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