quay-pruner: completely overhaul prune-quay.py

quay-pruner: completely overhaul prune-quay.py

Pruning had stopped working (up to >26000 image tags), and the
reasons were many; one, pruning's always been less deterministic
than I'd hope; two, when I switched us to ceph.git/container for
building images, I mistakenly changed the format of the 'fulltag'
(no longer has a short sha1 in it) and that was sort of driving
the pruning process.  three, I suspect some of the newer flavors
etc. were slipping through the cracks.

So here's an attempt to fix all that by changing the algorithm
fundamentally; now, tags of a certain manifest digest are considered
at the same time, and their sha1 checked in shaman as usual (but
only their sha1); if it's found, the tags are all left, and if not,
they're all removed.  This should be cleaner, faster, and more
reliable.

Also refactored a lot of the worker routines to util.py so I could
add some helper/debug/info scripts:

get-tagdates.py generates JSON showing tag-to-age for examining the
state of things

delete-tags.py takes tags on the CLI to delete, or can be invoked
with '--stragglers <age>' to remove anything older than age (as long
as it's not in shaman or seems like it might be a 'distinguished' build
(with recent release names in its name)).

prune-quay.py also now reports summary statistics of its operation.