Move get_tmp_dir to lcb_get_tmpdir()

Makes it more testable!

Change-Id: I6572924e64c5f4673db6ffcedd450adf11d5ec31
Reviewed-on: http://review.couchbase.org/42548
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>