pim-msdp: part-3: use SA cache for setting up SPTs

pim-msdp: part-3: use SA cache for setting up SPTs

1. Added a new MSDP source reference flag for creating (S,G) entries
based on the SA-cache. The RFC recommends treating as SA like rxing
a (S, G) join (which is a bit different then treating like a traffic
stream).
2. SA-SPT is only setup if we are RP for the group and a corresponding
(*,G) exists with a non-empty OIL.
3. When an SA is moved we need to let the SPT live if it is active (this
change will come in a subsequent CL).

Testing done:
1. SA first; SPT setup whenever (*, G) comes around.
2. (*, G) first. As soon as SA is added SPT is setup.
3. (*, G) del with valid SA entries around.

Ticket: CM-13306

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>