datastore: UNION ALL for mysql list entries query

We have seen high CPU on mysql DB. Optimizing this frequent query should
help. This brings the mysql 8 and above that supports WITH clause on par
with postgresql. Essentially port
#4111 to
buildFetchRegistrationEntriesQueryMySQLCTE.

Signed-off-by: ztrain <ztrain@uber.com>

Author: zmt

pkg/server/datastore/sqlstore/sqlstore.go

@@ -2931,7 +2931,7 @@ FROM
 	registered_entries
 WHERE id IN (SELECT id FROM listing)
 
-UNION
+UNION ALL
 
 SELECT
 	F.registered_entry_id, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, B.trust_domain, NULL, NULL, NULL, NULL, NULL
@@ -2944,15 +2944,15 @@ ON
 WHERE
 	F.registered_entry_id IN (SELECT id FROM listing)
 
-UNION
+UNION ALL
 
 SELECT
 	registered_entry_id, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, id, value, NULL, NULL, NULL
 FROM
 	dns_names
 WHERE registered_entry_id IN (SELECT id FROM listing)
 
-UNION
+UNION ALL
 
 SELECT
 	registered_entry_id, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, id, type, value, NULL, NULL, NULL, NULL, NULL, NULL