Refactor the failover log

This change should make things simpler and also adds some
documentation to the failover log header file.

Change-Id: I0b69465f56efb604f97cccf3692217302420d380
Reviewed-on: http://review.couchbase.org/32822
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>