nat: configurable handoff frame queue size

nat: configurable handoff frame queue size

Make number of worker handoff frame queue elements configurable as
a set nat frame-queue-nelts command. The default value is 64 which
is the same value that was previously hard-coded. The idea is that
allowing larger values can be useful in some cases, to avoid
congestion drops. Also add nat_set_fq_options API support and a
corresponding test case.

Type: improvement

Change-Id: I5c321eb2d7997f76fac2703d9c4a5b2516375db3
Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>