Adding a simple ready only query cache

[kreuzhofer]

The read only query cache boosts the read performance if you have large read only databases like catalogs that client users browse and search frequently. We currently use this feature in a mobile app, which has 1 million products in a database. Performance has improved drastically since we use it. When we update the db, we turn off the cache. Though the cache is automatically cleared if any write operation is done.
I will add a cache limit, soon, that throws out the oldest entries once the limit is reached. This is for memory savings only and cache size will be configurable.