We have a number of people starting up singleton threads for the sole
purpose of running a single runnable on them. These consumers often
leave the thread running until some point close to shutdown, or they
never shut it down at all. Let's add a helper function to do the thing
they actually want to do, and then we can modify the implementation of
that function as necessary as we merge singleton threads (and even
thread pools) together.
Differential Revision: https://phabricator.services.mozilla.com/D46997
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