Use failover_entry_t struct instead of entry_t

entry_t is a std::pair and we can be more descriptive by just
declaring a struct for a failover log entry. We should also define
this new struct in a place that allows us to pass failover entries
around.

Change-Id: Ie361290bfc16b3e768ed41279bab87979ba0b4f2
Reviewed-on: http://review.couchbase.org/32755
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>