(dev/core#635) Only write extra prevnext cache record if used

(dev/core#635) Only write extra prevnext cache record if used

This line of code creates an extra record to facilitate clearing the
SQL-based prevnext cache.  However, with Redis-based prevnext cache, it's
not needed because the Redis implementation simply uses a TTL -- and (on
Redis deployments) this lines to gratuitous SQL writes.