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.
Cool, that was kind of bothering me but making a new sys prop just to trace this one thing seemed excessive and unlikely to get used.  Good to know there's a more general one already.