ifnotifier: do not wake up when there is no db connection

ifnotifier: do not wake up when there is no db connection

When bridge uses the interface notifier, it wakes up until a reconfiguration
takes place. However, if there is no connection or a lock contention to the
database, the check for reconfiguration will not take place.

This uses a seq and only seq_wait when checking for the interfaces change.

This is easily reproduced by starting ovs-vswitchd without starting
ovsdb-server, and then creating a new system interface, like using
'ip link add type veth'. ovs-vswitchd will then consume 100% CPU.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>