Granular link health statistics for cfm.

The changes display the cfm_health of an interface.  The cfm_health
is an exponential weighted moving average of the health of all
remote_mpids.  The value can vary from 0 to 100, 100 being very healthy
and 0 being unhealthy.

Feature #10363
Requested-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Mehak Mahajan <mmahajan@nicira.com>