Tahoe-LAFS 1.17.1 StorageServer is not compatible with our usage

In Tahoe-LAFS 1.17.1 allmydata.storage.server.StorageServer no longer implements RIStorageServer. Instead, it implements an unnamed interface which can be used as the protocol-agnostic backend for an RIStorageServer implementation.

The RIStorageServer implementation is now at allmydata.storage.server.FoolscapStorageServer.

ZKAPAuthorizer needs to be adjusted to account for this change.