Fix async queue teardown

[AnastaZIuk]

Summary

Fix async queue teardown by draining accepted work before request storage is destroyed.

Root cause

The lifecycle issue comes from 9414a480ffe, where IThreadHandler started stopping its worker thread from the base destructor. For CRTP queue users this is too late because derived resources and request storage are destroyed first.

Later async queue changes in de28767cc1, 418db0d4c1, and 87eeacb250 kept the same lifetime model while request slots started carrying associated futures and cancelled requests relied on the worker for cleanup. That made the old teardown ordering visible as an intermittent Debug assert when request_pool was destroyed before all slots were recycled.

This matches the random failure seen around #1079. It is a queue lifecycle bug, not a DXC compile failure.

Fix

Queue owners now call shutdown() from their own destructors. Shutdown closes submissions, waits for active request() calls to finish publishing, drains pending and cancelled slots, and only then stops the worker thread.