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.
Be nice if you could follow common code conventions here and add curly braces to these if statements. As well typically folks add some space around use of comparison operators.