regression: Ensure BoardListWatch gRPC call correctly release resources

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

It's a bugfix to a regression: the gRPC API call BoardListWatch kept the packagemanager locked until the watcher was closed.

What is the current behavior?

What is the new behavior?

The packagemanager is released right after the board watcher feed is obtained.

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.24%. Comparing base (cbec871) to head (a410977).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2681   +/-   ##
=======================================
  Coverage   70.23%   70.24%           
=======================================
  Files         222      222           
  Lines       21348    21345    -3     
=======================================
  Hits        14993    14993           
+ Misses       5176     5174    -2     
+ Partials     1179     1178    -1     

Flag	Coverage Δ
unit	70.24% <100.00%> (+<0.01%)	⬆️

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