Use a consistent UDP src port per "session"
Use a consistent UDP src port per "session"

This helps particularly when debugging the flow records with
wireshark, since wireshark won't match the templates with data
unless the UDP src port is the same.

alexandergall/snabbswitch,snabbco/snabb,dpino/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,dpino/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,eugeneia/snabbswitch,Igalia/snabb,snabbco/snabb,Igalia/snabbswitch,Igalia/snabb,Igalia/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,alexandergall/snabbswitch,eugeneia/snabbswitch,eugeneia/snabb,eugeneia/snabbswitch,Igalia/snabb,Igalia/snabb,dpino/snabb,dpino/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,eugeneia/snabb,snabbco/snabb,Igalia/snabb,dpino/snabb,snabbco/snabb,snabbco/snabb,dpino/snabb,eugeneia/snabb,snabbco/snabb,Igalia/snabbswitch,eugeneia/snabb,SnabbCo/snabbswitch,eugeneia/snabbswitch,eugeneia/snabb,dpino/snabb,eugeneia/snabb,SnabbCo/snabbswitch,alexandergall/snabbswitch,dpino/snabb,snabbco/snabb,Igalia/snabb,eugeneia/snabb,SnabbCo/snabbswitch,dpino/snabbswitch,dpino/snabbswitch,dpino/snabb,Igalia/snabbswitch