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/include/common/defaults.h b/include/common/defaults.h
index f53c611..f5c74db 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -135,6 +135,11 @@
 #define MAX_POLL_EVENTS 200
 #endif
 
+// the max number of tasks to run at once
+#ifndef RUNQUEUE_DEPTH
+#define RUNQUEUE_DEPTH 200
+#endif
+
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