Adds tcp_keepalive and tcp_keepidle config options

Currently the wsgi server will not close connections once requests
complete and will not enable keepalive on it's wsgi sockets. This can
be a problem for those who need to align the server keepalive with
load balancer timeouts without modifying system keepalive settings.

To remedy this we add new config options tcp_keepalive and
tcp_keepidle which are disabled by default to remain backwards
compatible.

DocImpact: "new config options for wsgi tcp_keepalive & tcp_keepidle"
Closes-bug: bug 1260406
Co-authored-by: Hirofumi Ichihara <ichihara.hirofumi@lab.ntt.co.jp>
Change-Id: Ic53402c57e1ebe44cde4c18e5e15200dcbbcb04b
Merge "Adds tcp_keepalive and tcp_keepidle config options"