Optimize joint state broadcaster execution time

[saikishor]

Hello!

When testing the joint state broadcaster on high-frequency systems, it is observed that it is taking around 120-140 microseconds of execution, and it doesn't improve even when we choose to set the parameter publish_dynamic_joint_states: false . So, upon closer look, it seems like it always does the whole map lookup of the interfaces and it is consuming quite some time. When running 2 of these broadcaster it consumes half of our cycle time.

This PR proposes minor improvements that reduce the execution time by 70%

With the current state:

With the proposed fix:

Tested on TALOS robot by setting parameter publish_dynamic_joint_states: false

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.77%. Comparing base (f72f813) to head (3b830fa).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
..._state_broadcaster/src/joint_state_broadcaster.cpp	75.00%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2159      +/-   ##
==========================================
- Coverage   84.79%   84.77%   -0.03%     
==========================================
  Files         151      151              
  Lines       14833    14846      +13     
  Branches     1286     1289       +3     
==========================================
+ Hits        12578    12586       +8     
- Misses       1784     1788       +4     
- Partials      471      472       +1     

Flag	Coverage Δ
unittests	84.77% <75.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...oint_state_broadcaster/joint_state_broadcaster.hpp	100.00% <ø> (ø)
..._state_broadcaster/src/joint_state_broadcaster.cpp	85.85% <75.00%> (-1.71%)	⬇️

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[thedevmystic]

Sorry for nitpicking :)

Suggested change

	// Pre-computed pointers to name_if_value_mapping_ values for joint state interfaces
	// Caches pointers to name_if_value_mapping_ values for joint state interfaces

Here I think caches suits better.