Cleanly shutdown the GC messages task

[JamesWrigley]

start_gc_msgs_task() previously launched a lingering task that could cause hangs when Julia is started with multiple threads.

I'm hoping it'll fix hangs like this, which show that the task is stuck at the condition lock: https://buildkite.com/julialang/julia-master/builds/56286/steps/canvas?sid=019d66f2-8543-41d2-af4b-c14a111fed86

Change ported from JuliaParallel/DistributedNext.jl#62. @DilumAluthge, I think we should bump the Distributed version in Julia after this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (1a25fc7) to head (986a664).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   79.47%   79.56%   +0.08%     
==========================================
  Files          10       10              
  Lines        1959     1967       +8     
==========================================
+ Hits         1557     1565       +8     
  Misses        402      402              

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