Merge branch 'ps/reftable-concurrent-compaction'

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