Keepalive packets were only sent every KeepaliveInterval if the socket

Keepalive packets were only sent every KeepaliveInterval if the socket
had been completely idle during that interval.
If we had been sending other packets such as Messages, Calls or Controls
there wouldnt be any need for an explicit keepalive and thus we didnt
send one.

This does make it somewhat awkward when analyzing traces since it is
non-intuitive when keepalives are sent and when they are not sent.

Change the keepalive logic to always send a keepalive regardless of
whether the link is idle or not.