bridge: Downgrade log level of some log messages that may indicate races.

bridge: Downgrade log level of some log messages that may indicate races.

Some legitimate system activity can cause network devices to be destroyed
just before they are removed from the database, added to the database
just before they are created, or destroyed and then recreated under the
same name while other database activity is going on.  Logging these
events as errors makes it sound like something unexpectedly bad is going
on, but in fact these events are the most common instances of these log
messages, so downgrade them to warnings.

Reported-by: Reid Price <reid@nicira.com>
Bug #2584.