MINOR: tasks: Make the number of tasks to run at once configurable.

Instead of hardcoding 200, make the number of tasks to be run configurable
using tune.runqueue-depth. 200 is still the default.
diff --git a/src/haproxy.c b/src/haproxy.c
index 6fd2e83..4e61e40 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1853,6 +1853,9 @@
 	if (global.tune.maxpollevents <= 0)
 		global.tune.maxpollevents = MAX_POLL_EVENTS;
 
+	if (global.tune.runqueue_depth <= 0)
+		global.tune.runqueue_depth = RUNQUEUE_DEPTH;
+
 	if (global.tune.recv_enough == 0)
 		global.tune.recv_enough = MIN_RECV_AT_ONCE_ENOUGH;