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.

First cut of thread pools in fshell. Not fully working yet. Removed 4 overloads of CCommandBase::RunCommand[L] that are no longer used at all, and changed one more to not be exported as it's only used internally to iocli.dll. Added a thread pool (CThreadPool) to fshell so that thread commands no longer have to spawn a new thread every time they run. As part of this, CCommandWrapperBase now derives from CActive so that CThreadCommand can use its CActive-ness. CThreadCommand rejigged quite a bit to use CThreadPool (via MTaskRunner interface).