session: cleanup session tx function

session: cleanup session tx function

- rework the function to declutter and avoid building more than one tx
  frame
- add dual loop although benefits in my tests seem to be minimal
- improve tcp/udp echo external apps. They have slightly better
  throughput than internal echo apps.
- udp bugfixes

Change-Id: Iea4a245b1b1bb407a7f403dedcce2664a49f774b
Signed-off-by: Florin Coras <fcoras@cisco.com>