PIMDM: last fixes of the downstream state machine

- when PruneTimer expires, and upstream was pruned, then send Graft message upstream
- memory leak in multicastReceiverRemoved() and rpfInterfaceHasChanged()
PIMDM: StateRefresh fixes

- added explicit state variable for originationState
- setting the TTL field of StateRefresh messages
- StateRefresh messages are sent to ALL_PIM_ROUTERS_MCAST address instead of the group address
- do not send StateRefresh on interfaces where the router is assert looser
- SourceActiveTimer is only used in the router directly connected to the source