Fix threading issue for collision velocity scaling in MoveIt Servo

[eholum]

Description

Fixes a thread safety issues in the collision monitor in MoveIt Servo. Basically, the shared atomic double collision_velocity_scale_ was being reset to 1.0 at the start of each collision check loop, which would sometimes be picked up in the Servo::getNextJointState, so the robot ends up moving even though the scene is in collision. E.g.,

[component_container_mt-7] ERH TEMP: Servo::getNextJointState, collision_velocity_scale_=1                                                                                                    
[component_container_mt-7] ERH TEMP: CollisionMonitor::checkCollisions, collision_velocity_scale_=0      

To fix this, we should use a local variable to compute the new scaling factor, and only set the shared variable at the end of the collision processing loop.

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

[sea-bass]

@ibrahiminfinite another one! I think I might have caused this one out of negligence in some of my earlier "fixes".

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f9ae31b) 50.79% compared to head (4518c96) 50.80%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2517      +/-   ##
==========================================
+ Coverage   50.79%   50.80%   +0.01%     
==========================================
  Files         392      392              
  Lines       32168    32168              
==========================================
+ Hits        16338    16341       +3     
+ Misses      15830    15827       -3     

Files	Coverage Δ
moveit_core/utils/src/logger.cpp	100.00% <100.00%> (ø)
moveit_ros/moveit_servo/src/collision_monitor.cpp	89.10% <0.00%> (-1.81%)	⬇️

... and 2 files with indirect coverage changes

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[sea-bass]

Thank you!