Zebra: adding support for Zebra MLAG Functionality

Zebra: adding support for Zebra MLAG Functionality

This includes:
1. Processing client Registrations for MLAG
2. storing client Interests for MLAG updates
3. Opening communication channel to MLAG  with First client reg
4. Closing Communication channel with last client De-reg
5. Spawning a new thread for handling MLAG updates peocessing
6. adding Test code
7. advertising MLAG Updates to clients based on their interests

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