SL-16421: Destroy the "General" ThreadPool as soon as cleanup starts.

Introduce LLAppViewer::onCleanup(), a method that accepts a nullary callable
to execute once viewer shutdown begins. Fire the collected callables in
LLAppViewer::cleanup().

In llstartup.cpp, instead of declaring a static unique_ptr and relying on
static object destruction to clean up the "General" ThreadPool, bind the
pointer to the new ThreadPool into an onCleanup() lambda that will delete it
when called. ~ThreadPool() takes care of orderly shutdown.

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