NG-MVPN: PIM Sparse Mode (SM)

This feature adds support for PIM sparse mode (PIM SM) for next-generation MVPN. PIM sparse mode uses reverse-path forwarding (RPF) to create a path from a data source to the receiver requesting the data. In this mode, the assumption is that very few receivers want packets from a source. Thus, the source device establishes and sends packets only on branches that have at least one leaf that has requested for data.