Investigate statement close behaviour after connection close

[mrotteveel]

Some spurious test failures in the hibernate-orm tests suggests that attempts to close a statement after the connection has been closed may result in error "The connection is closed [SQLState:08006, ISC error code:337248336]"; technically that is valid, but it does suggest that a connection close doesn't always close the associated statements.

This may be some exceptional circumstance, but verify this to be sure.