Update the hold timer when a BGP Update message is received
Update the hold timer when a BGP Update message is received

According to RFC4271,
       The calculated value indicates the maximum number of
         seconds that may elapse between the receipt of successive
         KEEPALIVE and/or UPDATE messages from the sender.

This change will reset the hold timer in the update FSM case.
(note, I am super new with Go. I'm not quite sure how to craft a good test case for this.)

a16/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,osrg/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp