Make the loss / delay sim available as an output feature

Add binary api and debug cli support.

Rewrite for speed: enqueue vlib_buffer_t's to the wheel, instead of
memcpy'ing data. Quad-loop the output feature / x-connect (interior)
node. Prefetch wheel entries in the input node.

Save packet-generator-based unit-test setup in extras/nsim.

Simple config example:

set nsim delay 20 ms bandwidth 1 gbit packet-size 1024
nsim output-feature enable-disable GigabitEthernet3/0/0

Change-Id: I852a32d4eb596e7e2aa1d9b30bf3b53525e39fd1
Signed-off-by: Dave Barach <dave@barachs.net>c