[Bro] connection_status_update for inactive flows

[Siwek, Jonathan Luke]

> You could implement your script like this…
> 
> module MyModule;
> 
> export {
> 	## The period of delay for all established connections
> 	## before rechecking them for whatever I'm checking them for.
> 	const checkup_interval = 5sec;
> }
> 
> event MyModule::regular_check(c: connection)
> 	{
> 	# Do your check you would have previously done in connection_status_update
> 	print c$uid;
> 
> 	# Reschedule this event.
> 	schedule checkup_interval { MyModule::regular_check(c) };
> 	}
> 
> event connection_established(c: connection)
> 	{
> 	# Schedule the event that does the check.
> 	schedule checkup_interval { MyModule::regular_check(c) };
> 	}

There's also a general form of connection polling provided by ConnPolling::watch() in scripts/base/protocols/conn/polling.bro, which allows the "checkup" interval to vary between connections and/or over time.

scripts/base/protocols/ftp/gridftp.bro has an example usage.

    Jon