Set DataLoader names when registering them

[bclozel]

As of graphql-java/java-dataloader#193, DataLoader are named.

We need to reflect this new feature in our codebase by:

1. Fill the new name property when creating a DataLoader from a ReactorBatchLoader/ReactorMappedBatchLoader using its name
2. Use this name property for the data loader observations, replacing the graphql.loader.type