Give processor_t more control over the lifecycle of a job

Jobs are now destroyed by the processor, but they are allowed to
reschedule themselves.  That is, parts of the reschedule functionality
already provided by callback_job_t is moved to the processor.  Not yet
fully supported is JOB_REQUEUE_DIRECT and canceling jobs.

Note: job_t.destroy() is now called not only for queued jobs but also
after execution or cancellation of jobs.  job_t.status can be used to
decide what to do in said method.

src/libstrongswan/processing/processor.h

@@ -51,7 +51,7 @@ struct processor_t {
 	/**
 	 * Get the number of threads currently working, per priority class.
 	 *
-	 * @param				prioritiy to check
+	 * @param				priority to check
 	 * @return				number of threads in priority working
 	 */
 	u_int (*get_working_threads)(processor_t *this, job_priority_t prio);