bfd/cfm: Check status change before update status to database.

This commit adds boolean flag in bfd/cfm module for checking
status change.  If there is no status change, the current
update to OVS database will skip the bfd/cfm session.

In the experiment with 5K bfd sessions, when one session is
flapping at rate of every 0.3 second, this patch reduces the
cpu utilization of the ovs-vswitchd thread from 13 to 6.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>