isodate duration parser has some bugs

Turn off implicit lease renewal for allocate_buckets

Make better decisions about lease renewal

so don't expect to see empty lists in the tests

This means the test suite can actually exercise logic that depends on
different lease expiration times for different shares in one bucket.

now that the server actually returns different info for different shares, we
can, and we should, and we must

Update to a version of tahoe that includes openmetrics support.

I dunno if this is the right solution but it makes the build work again.
Hopefully upstream will come up with a good fix of their own.