Using malloc/free for the http fields hash.

This is required due to lack of a graceful shutdown: there is a small gap
between the runtime's memory pool release and router process's exit. Thus, a
worker thread may start processing a request between these two operations,
which may result in an http fields hash access and subsequent crash.

To simplify issue reproduction, it makes sense to add a 2 sec sleep before
exit() in nxt_runtime_exit().

src/nxt_router.c

@@ -303,7 +303,7 @@ nxt_router_start(nxt_task_t *task, void *data)
     }
 #endif
 
-    ret = nxt_http_init(task, rt);
+    ret = nxt_http_init(task);
     if (nxt_slow_path(ret != NXT_OK)) {
         return ret;
     }