SL-17483: Recast LLImageDecodeThread as a facade for ThreadPool.

Remove all references to LLQueuedThread (but emulate a couple bits of its API
such as handle_t and getPending()).

Migrate ImageRequest into llimageworker.cpp. It has never been part of
LLImageDecodeThread's public API. Remove ImageRequest tests.

Remove all references to LLImageDecodeThread::pause(). The idea of pausing
another thread is bizarre to me, and LLThreadPool has no such operation. Nor
does it have an abortRequest().