Merge branch 'ps/reftable-stack-tempfile'

The code in reftable backend that creates new table files works
better with the tempfile framework to avoid leaving cruft after a
failure.

* ps/reftable-stack-tempfile:
  reftable/stack: register compacted tables as tempfiles
  reftable/stack: register lockfiles during compaction
  reftable/stack: register new tables as tempfiles
  lockfile: report when rollback fails