ENG-4524: Fix missing table block on snapshot.

If the very last tuple in a table returned by the CopyOnWriteIterator happens to
fill up the output buffer, the table block that contains that tuple will not be
returned back to the table. This causes data to be missing. Doing a last call to
CopyOnWriteIterator::next() makes sure that the last table block is returned
before the CopyOnWriteContext is torn down.