ofproto: Warn about excessive rule counts in OpenFlow tables.

Frequently we've run into controller bugs which result in hundreds of
thousands, or even millions of rules being installed in an OpenFlow
table.  This isn't something trouble-shooters naturally think of to
check for, so it's nice to have a low rate warning message to hint at
the potential problem.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>