bridge: Refactor bridge_reconfigure().

bridge: Refactor bridge_reconfigure().

The existing bridge_reconfigure() implementation is suboptimal.
When adding lots of new ports, on every pass through the run loop
it allocates a bunch of "struct iface"s and "struct port"s, only to
destroy them when out of time.  Additionally, when there are errors
adding or deleting ports, it can fail to converge.  Instead it will
attempt and fail to add the same set of ports forever.

This patch rewrites bridge_reconfigure() using a new strategy.
Whenever the database changes, some initial bookkeeping is done,
and a list of future work is compiled.  The bridge begins whittling
down this list, and stops processing database changes until
finished.

Bug #10902.
Signed-off-by: Ethan Jackson <ethan@nicira.com>