Prep-work patch for congestion-control

This patch addresses the missing field in various data-structures to
track valuable information to implement the congestion-control
algorithms and manage sub-connections states.

It adds the possibility to queue up to 2 SACKs chunks when the connection
is not gapping.

At the same time, it pushes some variable/field renaming for better
readibility.

Change-Id: Idcc53512983456779600a75e78e21af078e46602
Signed-off-by: Marco Varlese <marco.varlese@suse.de>