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/worker.cc

@@ -582,6 +582,13 @@ void Worker::set_maxwait(unsigned int maxwait)
     this_unit.max_poll_sleep = maxwait;
 }
 
+// static
+int Worker::pick_worker_id()
+{
+    static int id_generator = 0;
+    return atomic_add(&id_generator, 1) % this_unit.n_workers;
+}
+
 bool Worker::post(Task* pTask, Semaphore* pSem, enum execute_mode_t mode)
 {
     // No logging here, function must be signal safe.