Client storage backend should be removed as a configurable backend

[kostko]

The storage.backend configuration option should only have an effect for storage nodes and should not be needed for client nodes (where it must currently be set to client). The storage backend init should also be moved under go/worker/storage and the configuration options under worker.storage.*.

Note that compute nodes already automatically use the correct storage backend and the client node should do the same automatically.

This would make it easier to run client nodes as there would be one less configuration parameter to misconfigure.