Load module earlier, at server-rewrite phase. (opentracing-contrib#85)

Subrequests begin their lives in the server-rewrite phase, so this
module is still loaded for subrequests.

opentracing/src/ngx_http_opentracing_module.cpp

@@ -169,7 +169,7 @@ static ngx_int_t opentracing_module_init(ngx_conf_t *cf) noexcept {
 
   // Add handlers to create tracing data.
   auto handler = static_cast<ngx_http_handler_pt *>(ngx_array_push(
-      &core_main_config->phases[NGX_HTTP_PREACCESS_PHASE].handlers));
+      &core_main_config->phases[NGX_HTTP_SERVER_REWRITE_PHASE].handlers));
   if (handler == nullptr) return NGX_ERROR;
   *handler = on_enter_block;