session: deschedule sessions with no data to send

session: deschedule sessions with no data to send

This ensures the scheduler always tracks sessions that are descheduled,
i.e., do not have events in the old io events list. When app retries to
send, clear descheduled flag and potentially the pacer.

Consequently, transports no longer need to reset the pacer when
sessions are rescheduled after a long app tx pause.

This also fixes a tcp bug whereby the pacer was reset too often when
snd_una was equal to snd_nxt as there was no way to distinguish betwen
app tx breaks and congestion.

Type: improvement

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: Id3cc6c98cd76299e15030e504380dcf3c04c5189