Fix sporadically failing failover log test

The issue here is that we need to read the failover log id/seq as
a 64-bit number otherwise we can get partial failures since reading
it as a 32-bit number can cause the id/seq to get truncated.

Change-Id: Ibc8e8547f101915171e44c759bfff5abf289228a
Reviewed-on: http://review.couchbase.org/32614
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>