nginx_url-shortener.erb: don't turn off error log
- we are using www-data to run nginx, so set limit accordingly
- gzip compression level to 1
- worker_connections to a more sane 20000
- worker_rlimit_nofile to 999999
- add proxy_read_timeout of 300 seconds
- add invalid_header as a possible proxy_next_upstream option
- change back keep alive to 512
- add proxy_connect_timeout of 75 s
- turn off tcp_tw_recycle for now
- increase tcp_keepalive_intvl and tcp_fin_timeout to 25 and 30 respectively