Scan consistency queries not working on system events seq advances

Queries using scan consistency (request_plus) on collections will never get to the required DCP state if a vbucket has its latest sequence number advanced due to a system event (e.g. some collection created/dropped).
To reproduce:
1. Create a bucket, scope, and a collection c1.
2. Insert a single record in c1.
3. Create Analytics collection on c1.
4. Run scan consistency query (request_plus) on c1.
The query will wait indefinitely if no scan_wait is specified.