Add support for State Refresh

[troglobit]

"prevents the periodic timeout of prune state in routers, greatly reducing the re-flooding of multicast traffic down the pruned branches that expire periodically. It also causes topology changes to be realised quicker than the traditional 3 minute timeout."

Relevant RFCs:

• https://tools.ietf.org/html/draft-ietf-pim-refresh-02
• https://tools.ietf.org/html/draft-ietf-pim-dm-new-v2-05#page-24