Support the MAXAGE option for CLIENT KILL

[gerzse]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
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Description of change

Issue #3154

The CLIENT KILL command has a new option, called MAXAGE, to kill clients older than a given age. Add support for this new option.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 91.72%. Comparing base (2f88840) to head (e1e95b5).
Report is 19 commits behind head on master.

Files	Patch %	Lines
tests/test_commands.py	37.50%	5 Missing ⚠️
redis/commands/core.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3187      +/-   ##
==========================================
- Coverage   91.84%   91.72%   -0.13%     
==========================================
  Files         128      128              
  Lines       33232    33338     +106     
==========================================
+ Hits        30523    30579      +56     
- Misses       2709     2759      +50     

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[earonesty]

tested, simple change