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.
Can we use the existing gt2.jdbc.trace system property to enable this? See JDBCFeatureReader for details.