pimd : Add support for MLAG Register & Un-register

pimd : Add support for MLAG Register & Un-register

when ever a FRR Client wants to send any data to another node
using MLAG Channel, uses below mechanisam.

1. sends  a MLAG Registration to zebra with interested messages that
   it is intended to receive from peer.
2. In response to this request, Zebra opens communication channel with
   MLAG. and also in Rx. diretion zebra forwards only those messages which
   client shown interest during registration
3. when client is no-longer interested in communicating with MLAG, client
   posts De-register to Zebra
4. if this is the last client which is interested for MLAG Communication,
   zebra closes the channel.

why PIM Needs MLAG Communication
================================
1. In general on LAN Networks elecetd DR will send the Join towards
   Multicast RP in case of a LHR and Register in case of FHR.
2. But in case DR Goes down, traffic will be re-converged only after
   the New DR is elected, but this can take time based on Hold Timer to
   detect the DR down.
3. this can be optimised by using MLAG Mecganisam.
4. and also Traffic can be forwarded more efficiently by knowing the cost
   towards RP using MLAG

Signed-off-by: Satheesh Kumar K <sathk@cumulusnetworks.com>