This adds CThreadPool::cancelJob() and cancelJobs() which can cancel a set of
jobs synchronously. These functions only return when the job was successfully
cancelled.
It tries to cancel the jobs as quickly as possible, skipping any callbacks on
CJob that were not yet called. A job that is already running can use
CJob::wasCancelled() to check if it should quit.
Signed-off-by: Uli Schlachter <psychon@znc.in>
