[HW_IF] Prepare the handles for async operations

[saikishor]

As with #1688 and #1689 there are not many changes in the handles.hpp file. I think this is the right moment to do it, to have straight away working handles in async. The proposed changes would work with zero overhead in synchronous operations.

Looking forward for your feedback!

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 14 lines in your changes missing coverage. Please review.

Project coverage is 86.70%. Comparing base (eb4c19d) to head (eb8a27b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...re_interface/include/hardware_interface/handle.hpp	80.48%	4 Missing and 4 partials ⚠️
...de/hardware_interface/loaned_command_interface.hpp	42.85%	2 Missing and 2 partials ⚠️
...lude/hardware_interface/loaned_state_interface.hpp	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1750      +/-   ##
==========================================
- Coverage   86.77%   86.70%   -0.08%     
==========================================
  Files         116      116              
  Lines       10703    10748      +45     
  Branches      981      988       +7     
==========================================
+ Hits         9288     9319      +31     
- Misses       1062     1069       +7     
- Partials      353      360       +7     

Flag	Coverage Δ
unittests	86.70% <73.07%> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
...lude/hardware_interface/loaned_state_interface.hpp	60.00% <50.00%> (-6.67%)	⬇️
...de/hardware_interface/loaned_command_interface.hpp	71.42% <42.85%> (-28.58%)	⬇️
...re_interface/include/hardware_interface/handle.hpp	88.23% <80.48%> (-11.77%)	⬇️

[saikishor]

I've just gone through the coverage report, the missing coverage lines should already be handled in the testing of #1489 and #1567