SerialContainer: Improve enumeration of solutions

[rhaschke]

TODOs:

• Directly lift solutions to avoid impression of starvation
  • Lift state priority updates from internal to external states
• Rewrite SolutionCollector to enumerate solutions lazily and in cost-sorted order?
• Allow limiting the number of solutions generated by a SerialContainer
• Count time for enumerating solutions (in onNewSolution) for the container, not the generating child. #729

[codecov]

Codecov Report

❌ Patch coverage is 5.73770% with 115 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.57%. Comparing base (edcb472) to head (a8d8aa5).

Files with missing lines	Patch %	Lines
demo/src/pick_place_task.cpp	0.00%	115 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
- Coverage   52.86%   52.57%   -0.28%     
==========================================
  Files         138      138              
  Lines       10630    10683      +53     
  Branches     1128     1133       +5     
==========================================
- Hits         5618     5616       -2     
- Misses       5004     5059      +55     
  Partials        8        8              

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