Create thread specific zombie queues

Because each thread has their own epoll file descriptor and only one
thread can process a DCB, it makes sense to move to a per thread zombie
queue. This removes one of the last restrictions on scalability.

server/core/gateway.cc

@@ -1941,6 +1941,7 @@ int main(int argc, char **argv)
     /* Init MaxScale poll system */
     poll_init();
 
+    dcb_global_init();
     /**
      * Init mysql thread context for main thread as well. Needed when users
      * are queried from backends.