reduce UDP payload size to 1232

This patch reduces the maximal payload of all UDP stream to 1232 bytes.
This value is computed from the minimal MTU requested by rfc 2460
to support IPv6.
This value ensures that common networks (going to IPv6) support
a large enough packet size to be efficient, but a minimal size to
not be fragmented. This last case has a great incidence on packet drop
and reordering and validated by heuristic.

Change-Id: Icee79bb25b2fdcc70fef3efb4527209f8309be61
Tuleap: #887