nat: add ipfix rate-limiter for nat44-ed, nat44-ei and nat64

nat: add ipfix rate-limiter for nat44-ed, nat44-ei and nat64

This prevents ipfix flood with the repeating events and allows
to enable nat64 max_session and max_bibs events. Also fix wrong
endian for det44 and nat64 ipfix tests, now should be fine with
extended tests enabled.
Max session per user event @ nat44-ei requires more precise rate
limiter per user address, probably with sparse vec, not handled.

Type: improvement
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: Ib20cc1ee3f81e7acc88a415fe83b4e2deae2a836