Add support for fast rexmit of RA on link transitions.

Add support for fast rexmit of RA on link transitions.

Ticket: CM-7076
Reviewed By: CCR-4568
Testing Done:

To allow for faster convergence, modify transmission of IPv6 RA so
that on link transitions(link up, change VRF, link add etc.), we
transmit RA fast to ensure faster convergence with BGP Unnumbered.
But we don't enable Fast RA if RA interval is in msecs as we've
fixed the fast RA interval to be 1s. We retransmit RA 4 times, once
each second on link events and when RA is first enabled.