Logging and assertions to identify premature disposal of JDBCDataStore.

Some logging and assertions I added while identifying GEOS-6575 https://jira.codehaus.org/browse/GEOS-6575

Makes it easier to figure out what's responsible for a failure due to a disposed DataStore.
createConnection currently checks for null and provides an explicit NPE. Kevin is capturing it here, but we still expect an exception out of the usual execution path.

I would rather capture it here than trust client code calling JDBCDataStore.getDataSource() to perform a null check.