fixed remote_clusters_info bucket gc-ing

Apparently sometimes replication doc keys are atoms and sometimes
binaries. Expecting only binaries caused us to omit replication docs,
thinking that buckets that are not used by replications when they are.

Change-Id: Ia888c0f09bd15d0143ee5a7ed6d511af52bff9f8
Reviewed-on: http://review.couchbase.org/40149
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>