Abort and clean up pending transactions when a database is [closed]

[brodycj]

Right now, the plugin still keeps some state (including transactions still in the queue) even when a database file is closed or deleted. If the same database file is created or otherwise opened again, at some point the old pending transactions may fire off. It would be better to abort the transactions and clean up the internal memory resources.

IMPLEMENTATION NOTES:

The following alternatives may be applied to determine when a db is closed and what pending transactions to abort:

• When all db handle objects for the same db have been closed (related to Closing one of multiple db objects for the same db file causes failure #209), then abort all pending transactions for the db; or
• when a single db handle object for a db is closed, then abort only the pending transactions for the closed db handle.

Also, if there is already a transaction in progress for a db that is closed or deleted (which has not been committed or aborted), then it should be possible to abort (and ROLLACK) the transaction.

[brodycj]

As I said in #367 (comment) it would become more complex to abort all pending transactions when deleting a database. Instead, we should simply fix this issue in case of closing a database, and tell the user to close the database before deleting it. If someone tries to delete a database while there are pending (TBD open?) transactions, we should simply call the error callback (or perhaps throw an exception).