Add IGMP snooping

[acooks]

Many of the use cases for jittertrap involve UDP multicast traffic. Some specific multicast monitoring tools would be a nice addition.

It would be useful to be able to

• track multicast sources,

• track subscribers,

• track igmp queriers and then

• show the state that a snooping table would/should have

• show who's subscribed to what

• show when mismatched timer configuration between subscriber and querier can result in drops