Fixes to tcp active opens and reception

- Properly initialize connection index
- Add new session to main hash table
- Drop packets that are completely in the past with respect to the rcv
  wnd (e.g., retransmits due to premature timer pops)

Change-Id: Ic0873018ff0a4c59e0913863dc7f0c0f822d5c34
Signed-off-by: Dave Barach <dbarach@cisco.com>
Signed-off-by: Florin Coras <fcoras@cisco.com>