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
Does the datastore need to be completly recreated then (i.e. if the metadata map changes)? Or is there a way to update one already in use?

As for why changing the feature type can alter any of the store parameter - I am not sure why this was introduced. Cutting down on the frequency of resetting the datastore is my goal with this patch.

Aside I have some customer feedback (after testing this patch) about both POST and PUT being effected. I will report back on the bug ticket if I learn more.