Check params before clearing dataStoreCache (GEOS-6575)

See https://jira.codehaus.org/browse/GEOS-6575 for discussion.

Initial review here https://github.com/geoserver/geoserver/pull/668
This might break sql views, whose definition is hosted in the metadata map.
Also, I don't see how changing a feature type can alter any of the store connection parameters?