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.
Are we positive that throwing an exception here by default is not going to have any ill side affects? Might be safer just check for tracing and dump a stack trace.