pimd: Reset neighbor hold time when we receive packet

pimd: Reset neighbor hold time when we receive packet

When we receive a packet from a neighbor, reset the
hold time as that we *know* that they are still
alive.

During heavy packet load, we were seeing cases
where neighbors were being reset because we
were timing out due to not processing the hello
packet in time.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>