bridge: Rate limit port creations and deletions.

bridge: Rate limit port creations and deletions.

In some datapaths, adding or deleting OpenFlow ports can take quite
a bit of time.  If there are lots of OpenFlow ports which needed to
be added in a run loop, this can cause Open vSwitch to lock up and
stop setting up flows while trying to catch up.  This patch lessons
the severity of the problem by only doing a few OpenFlow port adds
or deletions per run loop allowing other work to be done in
between.

Bug #10672.
Signed-off-by: Ethan Jackson <ethan@nicira.com>