DRILL-3732: Drill leaks memory if external sort hits out of disk space exception
- ExternalSort.mergeAndSpill() cleans all it's data in case an errors occurs while it's spilling to disk
- made BatchGroup AutoCloseable so it can easily be closed with AutoCloseables.close() if an error occurs
- added injection site while External sort is spilling to disk
- added unit test that forces a 2 batch query to spill to disk and injects an exception while it does so
this closes #147
