PRIMARY_IDX_CBO_STATS index should be created with replicas wherever possible

The GSI index PRIMARY_IDX_CBO_STATS created on N1QL_SYSTEM_BUCKET.N1QL_SYSTEM_SCOPE.N1QL_CBO_STATS whenever stats are collected for CBO should be created with index replicas if there are multiple indexer nodes. Without this, if the index resides on an indexer node that needs to be automatically failed over, the Index auto failover will not work as this index violates the constraint.

See https://issues.couchbase.com/browse/MB-49356?focusedCommentId=569639&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-569639 for more details.