cfm: Improve logging.

cfm: Improve logging.

This patch makes a two improvements to CFM logging which should
make debugging connectivity problems a bit more intuitive.  First,
when a remote_mp disappears, the length of time since its last CCM
reception is logged.  Second, the "CFM fault status changed"
message is reformatted in a more intuitive way.  Instead of
prefixing additions and deletions with pluses and minuses, the full
old fault status and new fault status are logged.

Requested-by: Ben Basler <bbasler@nicira.com>,
Signed-off-by: Ethan Jackson <ethan@nicira.com>