Addresses issues in page cache implementation

Now uses CMS_CACHE_PREFIX for page cache version item to prevent clashing with other projects using same cache-backend. Also, avoids an issue where the master version could expire before entries cached against it due to the use of cache.incr() (which doesn't advance the TTL).
Merge pull request #2804 from mkoistinen/fix_page_cache

Addresses issues in page cache implementation