Events: the "accept_mutex" directive is turned off by default.

Now it is believed that the accept mutex brings more harm than benefits.
Especially in various benchmarks it often results in situation where only
one worker grabs all connections.

src/event/ngx_event.c

@@ -1261,7 +1261,7 @@ ngx_event_core_init_conf(ngx_cycle_t *cycle, void *conf)
     ngx_conf_init_ptr_value(ecf->name, event_module->name->data);
 
     ngx_conf_init_value(ecf->multi_accept, 0);
-    ngx_conf_init_value(ecf->accept_mutex, 1);
+    ngx_conf_init_value(ecf->accept_mutex, 0);
     ngx_conf_init_msec_value(ecf->accept_mutex_delay, 500);
 
     return NGX_CONF_OK;