lib/commit: Reuse txn dir for tmpfiles

lib/commit: Reuse txn dir for tmpfiles

This closes a race condition I was seeing with `test-concurrency.py`. If we
don't have `O_TMPFILE` (or for symlinks) we'll create temporary files;
previously these would be subject to the date-based pruning because we set the
timestamp to 0 for objects.

Having our temporary files also in the txn staging dir ensures that they're
covered by the locking we do for that directory, and it's also generally cleaner
since the lifecycle of all the temporary data for a txn is in one place.

Closes: #1352
Approved by: dbnicholson