MLNX Agent: Process port_update notifications in the main agent loop

MLNX Agent: Process port_update notifications in the main agent loop

This patch changes the way mlnx agent process port_update notifications.
It does the same for the mlnx agent as was done for the ovs-agent in
I219c6bdf63b0b5e945b655677f9e28fa591f03cd.
Processing a port_update notification directly in the RPC
call may cause competition with the main RPC loop.
To prevent this problem, the actual process of ports updates is done
in the main RPC loop, whereas the RPC call merely adds the updated
port MAC address to a set of updated ports.
port_update notifications received within a single main loop iteration
will be coalesced and processed only once.

Closes-Bug: 1279655
Change-Id: I63dda60cb3cf171e5e9111a1ecf95e45e1d86362
Signed-off-by: Roey Chen <roeyc@mellanox.com>