feature request: upterm host should shutdown if no connection is established within a given time frame

[bavarianbidi]

It would be nice to configure upterm host command with some kind of a timeout flag where it's possible to shutdown the server if no connection got established within the given time period.

more context/details:

The current implementation (and your fork as well) currently are doing all the connection timeouts only on a connection layer in the HandleConnection func (https://github.com/gliderlabs/ssh/blob/master/server.go#L262).

I've added a comment to the generic timeout issue #11

slightly related is this PR for a new timeout config param: PR #204 - handshake timeout