MXS-1948: Do round-robin worker assignment

Due to the skewed accept distribution without SO_REUSEPORT, we use
round-robin assignment of workers for new client connections. This
provides better performance as work is more likely to be evenly
distributed across all threads.

Using a least-busy-worker algorithm would provide a more stable result but
this is not trivially simple to implement. For this reason, the
round-robin based approach was chosen for 2.2.

server/core/internal/worker.hh

@@ -495,6 +495,13 @@ public:
      */
     static void set_maxwait(unsigned int maxwait);
 
+    /**
+     * Get next Worker's ID
+     *
+     * @return The ID of the worker where work should be assigned
+     */
+    static int pick_worker_id();
+
 private:
     Worker(int id,
            int epoll_fd);