dhcp: ipv6 prefix delegation improvements

dhcp: ipv6 prefix delegation improvements

Autoconfigure router advertisements for delegated prefixes.

Clean up a longstanding issue. If vpp receives a dhcpv6 renew reply,
do NOT reset per-delegated-prefix timers. That prevented vpp from
sending a solicit to renew the delegation on time. That, in turn
caused the RA code to send advertisements with valid_time =
preferred_time = 0.  That causes almost any downstream client to throw
away its delegated address.

Miscellaneous changes

o src/vnet/ip/ip6_neighbor.c - always memset elements allocated from pools
  to zero. DGMS.
o Remove debug spew from the ipv6 connection-tracker plugin

Type: feature

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I428feccdc47efdc413898600e0d62916928a6eb7