Merge branch 'ps/reftable-concurrent-compaction'

The code path for compacting reftable files saw some bugfixes against concurrent operation. * ps/reftable-concurrent-compaction: reftable/stack: fix segfault when reload with reused readers fails reftable/stack: reorder swapping in the reloaded stack contents reftable/reader: keep readers alive during iteration reftable/reader: introduce refcounting reftable/stack: fix broken refnames in `write_n_ref_tables()` reftable/reader: inline `reader_close()` reftable/reader: inline `init_reader()` reftable/reader: rename `reftable_new_reader()` reftable/stack: inline `stack_compact_range_stats()` reftable/blocksource: drop malloc block source