Aggressively drop compression buffers in ORC OutStreams

In Hive 0.11, when ORC's OutStream's were flushed they dropped all of the their buffers. In the patch for HIVE-4324, we inadvertently changed that behavior so that one of the buffers is held on to. For queries with a lot of writers and thus under significant memory pressure this can have a significant impact on the memory usage.

Note that "hive.optimize.sort.dynamic.partition" avoids this problem by sorting on the dynamic partition key and thus only a single ORC writer is open at once. This will use memory more effectively and avoid creating ORC files with very small stripes, which will produce better downstream performance.