Support ServerAliveInterval

[russjones]

OpenSSH allows setting ServerAliveInterval which sends a keepalive@openssh.com request at the configured interval:

https://github.com/openssh/openssh-portable/blob/master/clientloop.c#L480-L493

At the moment, Teleport ignores these requests:

https://github.com/gravitational/teleport/blob/master/lib/sshutils/server.go#L268-L276
https://github.com/gravitational/teleport/blob/master/lib/srv/sshserver.go#L623-L626

Instead Teleport always replies at it's own interval:

https://github.com/gravitational/teleport/blob/master/lib/sshutils/server.go#L259-L263
https://github.com/gravitational/teleport/blob/master/lib/sshutils/server.go#L285-L288

When OpenSSH doesn't get a response back, it kills the connection which ends up having the opposite result of what the user intended.