VXGW: Preparation for feeding ARP-supression table.

This patch introduces changes in the handling of the
Ucast_Macs_Remote table in the vtep, so it offers enough
support to provide ARP-suppression functionality learned
from MidoNet.

It basically takes into account that several entries with
the same mac/ip/logical_switch may exist.

Ref: MN-1636

Signed-off-by: Ernest Artiaga <ernest@midokura.com>