Check params before clearing dataStoreCache (GEOS-6575)

https://jira.codehaus.org/browse/GEOS-6575
javadoc and log clarification of DataStoreCache use (GEOS-6575)

Commit logs DataAccess implementation during DataStoreCache dispose.

https://jira.codehaus.org/browse/GEOS-6575

Conflicts:
	src/main/src/main/java/org/geoserver/catalog/ResourcePool.java
Always reset DataAccess if sql_views used, or if last feature type deleted
Merge pull request #669 from jodygarnett/rp_param_check

Check params before clearing dataStoreCache (GEOS-6575)