Add failing list data files race test

This test shows what happens when listing the readable files from a
Bitcask directory. The list should not include the active write or merge
files. But if those change after the code fetches the name of the
current one, the listing will contain the new active write and/or merge
files, skipping the *previous* write or merge file. That would be bad.

The test uses meck to return the filename, then in one go write a new
one to simulate the race.
Fix race listing readable files

A race existed as demonstrated by the newly added
list_data_files_race_test test. When listing readable files from a
Bitcask directory for merging and such, the current write or merge
output files should never be listed along with the other files. This
could lead to merges trying to read active files while they are being
written.

Now, the code fetches the current merge and write files, lists the
directory contents, filtering those out, and checks again if the merge
and write file have changed. If they have, we try again, only succeeding
if no changes have happened in between.
Merge pull request #198 from basho/bugfix/list-files-race

Fix race listing readable files

Reviewed-by: slfritchie