Improvements to robustness of CThreadPool. Rationalised CThreadPool a bit, so that only the main thread is used for thread death notifications. This means that the death of parent threads no longer causes a panic when the child threads is shut down.

winscw \fshell\build\s60\3rd_edition winscw udeb
armv5 \fshell\build\s60\3rd_edition armv5 urel