status: report number of active worker threads in status output

This is especially useful when dynamic load management allocates more
worker threads than active tasks, so the extra threads are effectively
invisible.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/task.h

@@ -88,6 +88,9 @@ namespace crucible {
 		/// Gets the current number of queued Tasks
 		static size_t get_queue_count();
 
+		/// Gets the current number of active workers
+		static size_t get_thread_count();
+
 		/// Drop the current queue and discard new Tasks without
 		/// running them.  Currently executing tasks are not
 		/// affected (use set_thread_count(0) to wait for those