MB-20822: When loading persisted failovers, remove any erroneous entries

MB-20822: When loading persisted failovers, remove any erroneous entries

Due to bugs in older releases (and also possibly in future releases),
we may end up storing wrong failover table on disk. Hence during
warmup while loading failover table from disk we must prune out any
wrong entries.

Also, in the past we have seen erroneous entries with vb_uuid == 0.
So we make sure that vb_uuid is not generated as a valid value and
prune out any entry that has vb_uuid == 0

Change-Id: I630cb7fb1ea9a711432be64f36924d04fcd5e361
Reviewed-on: http://review.couchbase.org/67702
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>