session tcp: support pacer idle timeouts

session tcp: support pacer idle timeouts

Type: feature

To avoid excessive bursts, pacer must be provided with an estimated rtt
for the connection. That's used to compute an idle timeout, i.e., time
after which the bucket is reset to 1 mtu due to inactivity. For now,
idle timeout is computed as 5% of the rtt.

Change-Id: Ia0b752fe7b4ad0ce97b477fb886b0133a2321541
Signed-off-by: Florin Coras <fcoras@cisco.com>