pim-msdp: part-2: SA cache support

pim-msdp: part-2: SA cache support

This commit includes -
1. Maintaining SA cache with local and remote entries.
2. Local SA entries - there are two cases where we pick up these -
   - We are RP and got a source-register from the FHR.
   - We are RP and FHR and learnt a new directly connected source on a
     DR interface.
3. Local entries are pushed to peers immediately on addition and
   periodically. An immediate push is also done when peer session is
   established.
4. Remote SA entries - from other peers in the mesh group and passed
   peer-RPF checks.
5. Remote entries are aged out. No other way to del them
   currently. In the future we may add a knob to flush entries on
   peer-down.

Testing done -
Misc topologies with CL routers plus basic interop with another vendor (
we can process their SA updates and they ours).

Sample output -
root@rp:~# vtysh -c "show ip msdp sa"
Source                     Group               RP    Uptime
33.1.1.1               239.1.1.2            local  00:02:34
33.1.1.1               239.1.1.3            local  00:02:19
44.1.1.1               239.1.1.4        100.1.3.1  00:01:12
44.1.1.1               239.1.1.5        100.1.3.1  00:00:55
root@rp:~#

Ticket: CM-13306

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