shaper: add output port for packets that do meet the shaping condition

For example, you can do rate limited printing in pull context as
follows:
..
-> Shaper(10)
=> ( [0] -> Print -> [0];
     [1] -> [1] )
=> RoundRobinSched
-> ..

Also, add a reset handler for use by the testie that's part of this
change.

Reviewed-by: cliff@meraki.com
Signed-off-by: Patrick Verkaik <patrick@meraki.net>