Thread pools: memory barriers in task completion notifications.

The ngx_thread_pool_done object isn't volatile, and at least some
compilers assume that it is permitted to reorder modifications of
volatile and non-volatile objects.  Added appropriate ngx_memory_barrier()
calls to make sure all modifications will happen before the lock is released.

Reported by Mindaugas Rasiukevicius,
http://mailman.nginx.org/pipermail/nginx-devel/2016-April/008160.html.

src/core/ngx_thread_pool.c

diff --git a/src/core/ngx_thread_pool.c b/src/core/ngx_thread_pool.c
index 0353085..f3655aa 100644
--- a/src/core/ngx_thread_pool.c
+++ b/src/core/ngx_thread_pool.c
@@ -345,6 +345,8 @@
         *ngx_thread_pool_done.last = task;
         ngx_thread_pool_done.last = &task->next;
 
+        ngx_memory_barrier();
+
         ngx_unlock(&ngx_thread_pool_done_lock);
 
         (void) ngx_notify(ngx_thread_pool_handler);
@@ -366,6 +368,8 @@
     ngx_thread_pool_done.first = NULL;
     ngx_thread_pool_done.last = &ngx_thread_pool_done.first;
 
+    ngx_memory_barrier();
+
     ngx_unlock(&ngx_thread_pool_done_lock);
 
     while (task) {