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.  Behavior around file_sorter crashing is still the same as before (in an unchecked try), but decided that was acceptable - if it's due to perms problems on the tmpdir the process will crash in key_exchange instead.

Resolves #859