proposal: add option to not close idle session

[apmattil]

I'm making proxy and have tcp connections that are just opened but no data is sent.
If I enable keepalive, session is closed after 30 sec.

Could you add option not to close session if it has streams open ? not just data passed.
I do like to detect if connection is broken when keepalive's do not get reply.

I know this could be done at my proxy as well but implementing it to smux would make things much simpler.

[xtaci]

Config.KeepAliveDisabled just do the job