vlib: queue_hi_thresh fix to avoid deadlock

vlib: queue_hi_thresh fix to avoid deadlock

Adapt queue_hi_thresh value using num_threads to avoid risk of deadlock
between threads which could happen for example when different NAT
threads try to handoff work to each other at the same time when their
frame queues are congested. This change ensures that each thread can
reserve a queue entry without causing problems even in the most extreme
case when all threads attempt to add to the same queue simultaneously
when the queue is nearly full.

Type: fix

Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>
Change-Id: I9e02f753bd00833d8dd500d181b0d4f9a454d703