Destroy service clients explicitely

[christophfroehlich]

While trying to fix ros-controls/ros2_control_demos#666 I found out that the cm service_caller adds a client for every call, but never destroys it.
If the spawner is used with a list of controllers, it will create identical service clients in the same node -> might cause issues.

[saikishor]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.71%. Comparing base (7374c43) to head (0045673).

Files with missing lines	Patch %	Lines
.../controller_manager/controller_manager_services.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1944      +/-   ##
==========================================
- Coverage   87.73%   87.71%   -0.02%     
==========================================
  Files         122      122              
  Lines       13010    13012       +2     
  Branches     1165     1165              
==========================================
  Hits        11414    11414              
- Misses       1165     1166       +1     
- Partials      431      432       +1     

Flag	Coverage Δ
unittests	87.71% <50.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
.../controller_manager/controller_manager_services.py	79.01% <50.00%> (-0.37%)	⬇️

... and 1 file with indirect coverage changes

[bijoua29]

@christophfroehlich Looks good to me. Except I may have a related PR that may obviate the need for this PR. If my PR works, I will submit it within the next day. Either way, I will report here. Do you mind waiting a day before merging this PR? It's OK if you don't want to wait too.