Switch to new zkapauthorizer that works with NixOS 22.11.
Since we're not a flake yet we have to add a few work-arounds
as well.

See https://nixos.org/manual/nixos/stable/release-notes.html#sec-release-22.11-notable-changes
> The module services.grafana was refactored to be compliant with RFC 0042. [...]

Refs #129

Otherwise Tahoe might start without the socket unit and not have access to the
socket that's bound to the port and so just fail to start.

This reverts commit b8a88f35.

The current defaults already do not work.
Go back to set the dependencies manually.

It's safe to assume Systemd defaults are consistent and stay
the same.  Also, I know of a brilliant investment opportunity:
Mars is going to be colonized soon, and prices only can go up...

refs #122

This comes from nixpgks commit 81291cc793cf88bd6eff3fd8512e5eb9d037066c
and will be included with nixos 22.11.

... and use a smarter Prometheus query to combine the two.

This should implement my actual intentions: Alert when
backups run for longer than 3h, and the the repo check
for more than 6h.

Fixes privatestorageops#287

refs #122

Forgot to add a second alert when I added the workaround to
not count the ZFS ARC into used memory :/

... which is both governed by our retention policy.