ofproto: Store time since last connect and disconnect in Controller table.

ofproto: Store time since last connect and disconnect in Controller table.

ovs-vswitchd writes only the duration of its connection to or disconnection
from each controller to the database. This changes that behavior to write the
time since both the last connection and disconnection events regardless of
connection state. This mirrors the new behavior for reporting database manager
connection status.

Requested-by: Peter Balland <peter@nicira.com>
Bug #4833.