L3 multicast statistics support

We supported hit-bit to indicate the liveness of each multicast route. We can enable Layer 3 (L3) multicast statistics to track the total number of packets and bytes that hit a (S,G) multicast route. The routes can be a combination of IPv4 and IPv6 (S,G) routes. We support both any-source multicast (ASM) and source-specific multicast (SSM) routes. To enable multicast statistics use the mcast stats-enable statement at the [edit system packet-forwarding-options] hierarchy level.