bridge: Write certain statistics to the database instantly.

bridge: Write certain statistics to the database instantly.

Traditionally the bridge has written interface and port statistics
to the database in a rate limited fashion.  This makes a lot of
sense for statistics which are either constantly changing, or are
expensive to collect.  However, some statistics were rate limited
which have neither of these properties.  Furthermore some of these
statistics (most notably carrier) could be very useful to a
controller if updated promptly.

Signed-off-by: Ethan Jackson <ethan@nicira.com>