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
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