tcp/session: tx optimizations

- cache and reuse tcp options and rcv_wnd for session layer tx bursts
- avoid reading/setting total_length_not_including_first_buffer. It's
part of a buffer's second cache line so it comes at a "cost".

Change-Id: Id18219c2f7e07cf4c63ee74f9cdd9e5918904036
Signed-off-by: Florin Coras <fcoras@cisco.com>