reads all chunks in on-disk order and recompresses them if they are not already using
the desired compression type and level (and obfuscation level).
supports SIGINT/ctrl-c and --checkpoint-interval (default: 1800s).
this is a borg command that compacts when committing (without this, it would have
a huge space usage). it commits/compacts every checkpoint interval or when
pressing ctrl-c / receiving SIGINT.
also added some .pyi files needed to check the cython code (taken from #5703 and updated).
fixed "syntax error" in key.py.
all mypy complaints not fixed yet.