policer: add policer handoff

policer: add policer handoff

Add thread handoff for packets being policed. Note that the handoff
currently requires the policer index to be passed in. This is suitable
for use in the ip[46] punt paths where each policer node will only
ever use a single policer. For the more general case, this will be
expanded in future to use a policer index stored in packet metadata.

Type: improvement
Signed-off-by: Brian Russell <brian@graphiant.com>
Change-Id: I85a0ecbcfb025f8844e763224cd3de1561249aca