Random flows: Use seeds again and increase limit.

TRex does mix seeds when distributing over workers,
but it is multiplicative [0], so zero is the only bad value.

Limit restricts the cycle length of PRNG (by resetting [1] the seed).
We want the cycle as long as possible.

[0] https://github.com/cisco-system-traffic-generator/trex-core/blob/v2.73/src/stx/stl/trex_stl_stream_vm.h#L1616
[1] https://github.com/cisco-system-traffic-generator/trex-core/blob/v2.73/src/stx/stl/trex_stl_stream_vm.h#L313-L314

Change-Id: I33a29496f0853ef60d592c988f81a9d1109b5878
Signed-off-by: Vratko Polak <vrpolak@cisco.com>