Resolves GH #859 by setting tmpdir for sorting AAE disk logs.

Break out a function for determining the temporary directory
used by AAE and ensuring it's creation.  Pass the output from
the new tmp_dir() function to file_sorter.
Merge pull request #973 from basho/bugfix/gh859-aae-exchange-sortfile-tmpdir

Resolves GH #859 by setting tmpdir for sorting AAE disk logs.

Reviewed-by: russelldb