Add a cli for server load test

[justinlin-linkedin]

Summary

Create a tool to generate synthetic load to a single ambry-server instance for testing. It can generate all get, put, delete, update ttl traffic at the desired qps, either at the same time, or separately. It doesn't provide any client side metric so all the performance result has to be observed from server side metric.

Testing Done

Use this tool to test a single server.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 398 lines in your changes missing coverage. Please review.

Project coverage is 69.43%. Comparing base (52ba813) to head (9f9c939). Report is 253 commits behind head on master.

Files with missing lines	Patch %	Lines
...hub/ambry/tools/perf/ConcurrentServerLoadTest.java	0.00%	398 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3101      +/-   ##
============================================
+ Coverage     64.24%   69.43%   +5.18%     
- Complexity    10398    12538    +2140     
============================================
  Files           840      919      +79     
  Lines         71755    77911    +6156     
  Branches       8611     9309     +698     
============================================
+ Hits          46099    54097    +7998     
+ Misses        23004    20984    -2020     
- Partials       2652     2830     +178     

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[djycheng]

What's the difference between this test and the one we wrote with forge fire? Can we run the forgefire test on a one-node cluster if we wanted to verify performance of a single server?