fix(astf): avoid issues in pps tput

When more than 1 data packet is sent in the same chunk,
TRex is sometimes not fully deterministic
in its usage of delayed ACKs.

This changes the "application protocol" to sent
5 chunks (1 data packet each), c2s and s2c interleaved,
so each subsequent chunk acts as an ACK.

The overall packet count remains the same,
and even though this interleaved way may be more demanding
on TRex CPU, preliminary results show NAT performance
is still well below ip4base performance.

As a side effect, the interleaved way seems to work
also for 100B data frames, so we are avoiding two issues at once.

Ticket: CSIT-1846
Ticket: CSIT-1830

Change-Id: Ia4dcfa7c89f2c08fc32bd6118e2e009316b33c25
Signed-off-by: Vratko Polak <vrpolak@cisco.com>