bridge: Rate limit the statistics update.

bridge: Rate limit the statistics update.

When ovs is running with large topology (e.g. large number of
interfaces), the stats update to ovsdb becomes huge and normally
requires multiple run of ovsdb jsonrpc message processing loop to
consume.

To prevent the periodic stats update from backlogging in the
jsonrpc sending queue, this commit adds rate limiting logic
which only allows new update if the previous one is done.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Acked-by: Flavio Leitner <fbl@redhat.com>