Problem description: When autoflush is off and TTables are not closed after
transaction commit, a large number of cells without their corresponding
shadow cells may appear.
When the HBaseTransactionManager commits a TX, it:
1) writes the commit table entry for the Tx
2) updates shadow cells
3) erases the commit table entry
Before this fix, in step #2 the tables were not flushed, so some shadow cells
were not be written to HBase before the commit table entry was erased.