Fix several build request collapsing bugs

[p12tic]

This PR is a collection of 3 bug fixes in the default build request collapsing functionality. The collapsed build requests are not seen anywhere in the UI, so to the end users it would look like as if the schedulers were the broken piece.

Fixes #4686. Fixes #3762. Fixes #4642.

Contributor Checklist:

• I have updated the unit tests
• I have created a file in the master/buildbot/newsfragments directory (and read the README.txt in that directory)
• n/a I have updated the appropriate documentation

[codecov]

Codecov Report

Merging #6152 (4d6375c) into master (2c1428f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6152   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files         348      348           
  Lines       37315    37328   +13     
=======================================
+ Hits        34295    34308   +13     
  Misses       3020     3020           

Impacted Files	Coverage Δ
master/buildbot/db/model.py	92.06% <ø> (ø)
master/buildbot/process/buildrequest.py	96.08% <100.00%> (+0.30%)	⬆️

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[mwild1]

Thanks for this! ❤️