tcp: retransmit and multi-buffer segment fixes and improvements

- set session state as closed on session manager delete
- enable retransmit as opposed to persist timer after persist timer completes
- properly discard buffer chain bytes when new data overlaps ooo
  segments
- don't use rxt bytes in snd space estimate used on tx path

Change-Id: Id9cab686e532e5fe70c775d5440260e8eb890a9f
Signed-off-by: Florin Coras <fcoras@cisco.com>