Cleanly shutdown the GC messages task

`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 https://github.com/JuliaParallel/DistributedNext.jl/pull/62. @DilumAluthge, I think we should bump the Distributed version in Julia after this.