Merge branch 'ps/reftable-stack-compaction'

Merge branch 'ps/reftable-stack-compaction'

The code paths to compact multiple reftable files have been updated
to correctly deal with multiple compaction triggering at the same
time.

* ps/reftable-stack-compaction:
  reftable/stack: handle locked tables during auto-compaction
  reftable/stack: fix corruption on concurrent compaction
  reftable/stack: use lock_file when adding table to "tables.list"
  reftable/stack: do not die when fsyncing lock file files
  reftable/stack: simplify tracking of table locks
  reftable/stack: update stats on failed full compaction
  reftable/stack: test compaction with already-locked tables
  reftable/stack: extract function to setup stack with N tables
  reftable/stack: refactor function to gather table sizes