MB-10720 upr_replicator should process exists during blocking calls

Change-Id: Id4f489f895580ea41930a84c01621cebe55b0402
Reviewed-on: http://review.couchbase.org/35228
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>