as Tahoe now wants, so it can respect available space when allocating more for
a lease

The later function takes a count of passes to generated, and passes
them to `make_passes`. This also changes `make_passes` to return `Pass`es,
which is what `get_passes` wants to receive.

also rename and clarify the read_bucket `size` parameter

The test demonstrates this now by depending on part of the successful result
from the underlying storage server object.

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

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

This includes a minimal in-memory implemenation which can be used by tests, and
adds the appropriate calls to the new interface to the storage server
implemenation.

This means you can make estimates about storage-time remaining based on the
results of the calculator endpoint and they'll at least match your local
configuration.

also, move the function to the config module.

Also make the metrics path's parent directory if necessary