It's more specific and less powerful, therefore more expressive and safe.

Co-authored-by: Tom Prince <tom.prince@private.storage>

Also make the metrics path's parent directory if necessary

Let's see how far we get with just this simpler version.  We now have a
histogram for number of zkaps spent, bucketed by the size of individual spend
operations.

Also implement it for mutables

Add configuration for lease maintenance behavior

Fix the pyflakes invocation in Nix so it checks our source files.

Fold `PassValidationTests._test_lease_operation_fails_without_passes` into its caller.

It turns our that `pyflakes` with no arguments expects input
on stdin. In CI, this is likely connected to `/dev/null` so it
passes.

Also fix the errors that pyflakes is reporting.

It was orginally factored out since there were both `add_lease` and `renew_lease methods`.
However, since tahoe 1.16, the client no longer defines `renew_lease`, and we removed it
from our protocol in #231. Given that there is now only one caller, we can remove the
factoring.

isodate duration parser has some bugs