DRTVWR-476: Introduce LLCoprocedureManager::close(). Use in tests.

The new close(void) method simply acquires the logic from
~LLCoprocedureManager() (which now calls close()). It's useful, even if only
in test programs, to be able to shut down all existing LLCoprocedurePools
without having to name them individually -- and without having to destroy the
LLCoprocedureManager singleton instance. Deleting an LLSingleton should be
done only once per process, whereas test programs want to reset the
LLCoprocedureManager after each test.