fix plugin and middleware callback order for EntitiesField

[adaugherty42]

Absinthe.Middleware.Batch is both a plugin and a middleware. When used to resolve a non-federated field, absinthe calls its callbacks in the following order:

before_resolution (Plugin callback)
call (Middleware callback)
after_resolution (Plugin callback)
pipeline (Plugin callback)

The way Batch works is by setting up a data structure in before_resolution, accumulating the ids and setting the resolution state to suspended in call, and then calling the user's batch function in after_resolution. Finally, pipeline detects that the resolution is not complete, so it adds the Resolution phase back into the blueprint pipeline to be run again. When we go through the callbacks a second time, call hits a different pattern match because the state is now suspended, and it calls the user's post-batching function.

Right now EntitiesField is calling the middleware callback a second time before calling after_resolution, which is causing Batch to try to call the post-batching function before it has fired the actual batching function. Dataloader and Async are unaffected by this because they don't use after_resolution in the first place.

This PR shifts the callback order around to more closely match how absinthe resolves normal fields.

[kzlsakal]

Great contribution, thank you @adaugherty42!