fix regression from #1331

[mikeferguson]

Description

The noted PR swapped boost::thread to std::thread - however
std::thread will cause the program to crash with a note that
TERMINATE CALLED WITHOUT ACTIVE EXCEPTION if the thread object
goes out of scope while not joinable. Detaching the thread
restores the original workflow prior to the PR and allows
users to send more than one trajectory via the motion planning
plugin in RVIZ (otherwise, it crashes on the first trajectory).

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[tylerjw]

Thank you for this. I'm the future when we have jthread we could use that to fix this that way too.

[mikeferguson]

@tylerjw yeah - I'm not sure this is the best way to fix this - but it's definitely the simplest way to get things back to working

[codecov]

Codecov Report

Merging #1384 (dc0d45b) into main (038cb20) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1384      +/-   ##
==========================================
- Coverage   61.56%   61.55%   -0.01%     
==========================================
  Files         274      274              
  Lines       24977    24977              
==========================================
- Hits        15375    15371       -4     
- Misses       9602     9606       +4     

Impacted Files	Coverage Δ
...nning_scene_monitor/src/planning_scene_monitor.cpp	45.29% <0.00%> (-0.43%)	⬇️

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[tylerjw]

@tylerjw yeah - I'm not sure this is the best way to fix this - but it's definitely the simplest way to get things back to working

I don't understand how detached threads don't create a use after scope but it seems they are fairly common in ros code. I'm ok with it until someone has time to clean it up, definitely better than the bug you experienced.