Add toString for HiveOutputTableHandle
Make table owner required

Guava Optional cannot be used in handles because Guava is not part of
the SPI and thus they cannot be serialized due to being in a separate
class loader.
Remove unused class DataSourceType
Fix minor synchronization issue in TaskExecutor

The closed flag was not synchronized but is only used in tests
or when the server is shutdown.
Fix planning of TableCommit to run on coordinator
Fix closing of unpartitioned driver factories