Merge branch 'ps/reftable-optimize-io'

Merge branch 'ps/reftable-optimize-io'

Low-level I/O optimization for reftable.

* ps/reftable-optimize-io:
  reftable/stack: fix race in up-to-date check
  reftable/stack: unconditionally reload stack after commit
  reftable/blocksource: use mmap to read tables
  reftable/blocksource: refactor code to match our coding style
  reftable/stack: use stat info to avoid re-reading stack list
  reftable/stack: refactor reloading to use file descriptor
  reftable/stack: refactor stack reloading to have common exit path