Consider improving Reactor chain and context propagation

[ciscoo]

From #328 (comment):

(Emphasis mine)

Currently we decorate each DataFetcher and propagate to it Reactor context from the web layer. Apart from that they are not in a single Reactor chain and Reactor context from one cannot be seen by others. This is something we might try to improve in the future, but for now if you want pass something across data fetchers, just use GraphQLContext.

[bclozel]

I think that #459 might have solved this, as the chain of contexts (threadlocal, reactor context, graphql conetxt) is now consistent for the entire execution.

[rstoyanchev]

We do have improved context propagation after #459. However, Reactor DataFetchers are still not in a single chain of execution given that graphql.execution.AsyncExecutionStrategy expects each to return CompletableFuture and joins them as such for the final result. So the main way to pass context from one DataFetchers to others remains via GraphQLContext, which should be fine to use. I'm going to close this for now as there isn't anything further we intend to do here at this time.