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