Layering overhaul

- Collapse current front/backend layers into
    - _proxy layer_
        - global, per-domain and per-bucket type handlers
        - can make requests to any of the other proxy layers
        - precedence: domain > global > per-bucket > table storage
    - _storage layer_: table & queue storage backends
- clarification about error types
- tweaks to the declarative handler spec
- plan for doc merging