CJob: Even cancel finished jobs

When a job was cancelled after its runThread() method finished, but before the
main thread noticed this and reacted, we would just run runMain() before and
pretend the job finished normally.

However, with CModuleJob this means that runMain() might get called for a module
which is currently being destructed. This has bad effects with virtual functions
and thus causes problems. It's better to just really cancel the job instead.

Signed-off-by: Uli Schlachter <psychon@znc.in>
CThreadPool::cancelJobs(): Add a comment explaining the logic a bit

Signed-off-by: Uli Schlachter <psychon@znc.in>