Plack: change to use Starlet and optimize app 

Change to use Starlet for app server, Starlet is more stable and faster than Monoceros and Starman.
Optimize app.psgi. Don't use Plack::Builder and Plack::Response.

And use nginx as reverse proxy

Round 10 will use c3.large that has 2 cores.
I think `worker_processes auto;` is better.