Support GraphQL 1.6's multiplexed execution

[theorygeek]

In GraphQL 1.6, @rmosolgo introduced the Schema#multiplex feature 🎉 which lets you execute multiple queries in one go. But it seems that it invokes the query instrumentation once per query, instead of just once, which causes the NestedError to be raised. This change removes the NestedError behavior.

I'm opening this PR mainly as a discussion. I'm actually not sure what the ramifications are of allowing this nested behavior. But anywho, thoughts?

[rmosolgo]

I was thinking this might need a tiny bit more change, that the instrumenter could be adjusted to hook on instrument(:multiplex, ...), that way, the single executor could be shared by all queries.

I'd be happy to take it for a spin this afternoon, I think a proper test calling Schema#multiplex would be a good way to ensure that it works properly.

[rmosolgo]

Here's another take, what do you think?

#57

[theorygeek]

@rmosolgo Ah nice, yeah I think that's a better solution than the one I have here.