Support multiple engines, and stacking mode

[nevali]

Rather than servicing requests with a single engine, it would be desirable to enable support for multiple engines. For example, both spindle and file.

The "stacking mode" would allow a choice between “merge” (the request is passed to both engines) or “fallback” (where the request is only passed to subsequent engines if the previous engine in the stack returned a 4xx response).

[nevali]

See https://github.com/bbcarchdev/quilt/tree/QUILT-48

[cgueret]

Code available at https://github.com/bbcarchdev/quilt/tree/QUILT-48