Initial commit for IPV6 support in vrouter, more changes on the way. Current changes include

1) MTrie changes to support V6 prefixes. There will be 2 separate tables, 4-level for V4 and 16-level for v6. Stats are common for the time being, will separate the stats later
2) Common NH for V4 and V6. L2 rewrite code will update ethtybe.
3) MCast support yet to be added. Minimal changes in vr_mcast.c to allow V4 mcast to work
4) Sandesh changes to move rtr_prefix, rtr_marker and rtr_src in vr_route_req to LIST (instead of INT32) and related  changes throughout the code.
5) Flow support yet to be added. Minimal changes in vr_flow.c to allow V6 forwarding to work
6) Changes in vr_datapath.c and vr_proto_ip.c to allow V6 forwarding to work
7) 'rt' utils change to allow route add, del and dump

What works now:
0) V4/V6 route adds through rt utility
1) V4 ping between VMs on the same vrouter
2) V6 ping between VMs on the same vrouter
3) V4 route-dump

What doesn't work
0) V6 route dump
1) Agent doesn't start. With minimal changes, I was able to get the agent to build and start, but network connectivity gets lost completely

dp-core/vr_datapath.c
dp-core/vr_flow.c
dp-core/vr_ip4_mtrie.c
dp-core/vr_mcast.c
dp-core/vr_nexthop.c
dp-core/vr_proto_ip.c
dp-core/vr_route.c
include/vr_ip4_mtrie.h
include/vr_packet.h
include/vr_route.h
sandesh/vr.sandesh
utils/rt.c
Merge branch 'ipv6-devel2' of github.com:Juniper/contrail-dev-vrouter into ipv6-devel2