session: make group of 'static' vars more visible

session: make group of 'static' vars more visible

Put the collection of 'static' variables in datafeed_in() to the top of
the declaration block, to raise awareness during maintenance. These are
"semi global" variables with reduced visibility but extended lifetime.

Update a comment while I'm here. Explain what the early "header packet"
check does instead of what it doesn't.