upgraded the nginx core to 1.15.6.

patches/nginx-1.15.6-daemon_destroy_pool.patch

@@ -0,0 +1,12 @@
+diff --git a/src/os/unix/ngx_daemon.c b/src/os/unix/ngx_daemon.c
+index ab672110..f259af31 100644
+--- a/src/os/unix/ngx_daemon.c
++++ b/src/os/unix/ngx_daemon.c
+@@ -23,6 +23,8 @@ ngx_daemon(ngx_log_t *log)
+         break;
+
+     default:
++        /* just to make it ASAN or Valgrind clean */
++        ngx_destroy_pool(ngx_cycle->pool);
+         exit(0);
+     }