make reported max-mutable-share-size have the same semantics as max-immutable-share-size

[davidsarah]

Currently storage servers report max-immutable-share-size by consulting their available space and if configured their reserved_space configuration, but they report max-mutable-share-size as a hardcoded constant. This is inconsistent, and it is reporting less information to the storage client than it could, in the max-mutable-share-size field.

To close this ticket, make servers report max-mutable-share-size by consulting their available space.