perf(time): invoke ev_now_update for time.now()

When some event callback runs for a very long time without entering
the event loop, updating the current time is a good idea.

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

time.c

@@ -8,6 +8,7 @@
 static int eco_time_now(lua_State *L)
 {
     struct eco_context *ctx = luaL_checkudata(L, 1, ECO_CTX_MT);
+	ev_now_update(ctx->loop);
     lua_pushnumber(L, ev_now(ctx->loop));
     return 1;
 }