add keydir_trim_fstats to bitcask_nifs, allowing the post merge live
files update to remove entries for deleted files so they are no longer
considered for merge or for status
actually, um, iterate over the list
- add stats return for non-existent fstats entries
- log when non-existent fstats entries are trimmed
- dedup the fstats list so the above logging isn't constantly
  triggered.
Merge pull request #116 from basho/bugfix/pevm-fix-resource-leak

Fix fstat struct leak.