lazily create merge files to avoid creation of empty files

in situations with few hot keys, you can produce a whole ton of empty data files.  when all of keys in a partial merge have already expired, a data file will be created and nothing will ever be put into it.  These files aren't cleaned up until reopen, so they can cause problems when may of them accumulate over time.  

Moving to a lazy file creation strategy avoids this problem.

fixes #113