Add a cleanCompletion method to RadosFsIO

Also refactor the sync method to use this new one.
Replace sync with cleanCompletion in ~RadosFsIO

Because there is no reason why we should wait for the completion of
pending operations.
benchmark: Generate the options instead of hardcoding them
benchmark: Parse the new buffer size and division arguments
benchmark: Use the buffer size/division for writing files
benchmark: Add the new buffer size/division to the help output
benchmark: Fix the help output

The configuration argument was not correct.