Flexible batch

WIP to provide users with a comprehensive way to schedule tests as they see fit, rather than systematically defaulting to "all_packages_with_fastest_first".

The current HEAD aims at abckwards compatibility: nothing changes unless debci-batch is passed --external-sorter .

An example implementation of a flexible sorter is provided in tools/sort-by-priority.py, which can be used like this:

debci status --all --json | tools/sort-by-priority.py --priority '5="{package}".startswith("r")' --priority '40=re.search(r"kali","{version}")'

Ideally, all this code should be folded into debci-batch itself, but that can only happen after the latter is converted to python. If this whole "user-driven priorities" effort seems worthwhile, I can also take on that conversion.