Zebra: Handle FPM connection up/down events

Zebra: Handle FPM connection up/down events

- When the connection with the FPM socket is established, iterate through all the
  L3VNIs and send all the RMACs for FPM processing zfpm_conn_up_thread_cb"

- We have already handled connection down even in previous commits. When the FPM
  connection goes down, empty mac_q and FPM mac info hash table
  "zfpm_conn_down_thread_cb"

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>