Move Lazy resolution into Dataloader

[rmosolgo]

This is a moonshot but I want to see if the runtime code can be simplified by rebuilding lazy_resolve on top of GraphQL::Dataloader. Some hard requirements:

• No (or minimal...) performance hit for adopting Dataloader (not in speed or memory usage)
• Behavioral parity -- same batching performance and behavior
• Swappable per-query, so that you can try the new way or the old way while the app is running. This is essential for adoption: you have to be able to fall back to the old way if something goes wrong.

The reason I want to do this is because I want more flexibility in how GraphQL execution is started and stopped, so that I could reasonably support Ruby's lazy enumerators (#4907) and partial query execution (#5183).

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My todos:

• Make sure all existing legacy_sync_lazy(true) can be replaced with Dataloader
• Run builds with both legacy and Dataloader implementations
• Dataloader currently changes response key order. Have to fix that Dataloaders are changing the response order #4252
  ...
• Revisit API and write docs