PIMNeighborTable rewritten

- PIMNeighbor is an immutable class
- expiry logic moved from PIMSplitter to PIMNeighborTable
- livenessTimer is a simple cMessage instead of a dedicated class
- renames and comments
PIMNeighborTable: renamed methods
PIM: remove functionality from PIMSplitter that is unrelated to dispatching PIM packets
PIM: eliminated NF_IPv4_NEW_MULTICAST_DENSE and NF_IPv4_NEW_MULTICAST_SPARSE notifications