This requires a slightly newer graphql-extensions beta which has more arguments
to requestDidStart.
Also make it ok to not pass logFunction to formatApolloErrors, and make sure
custom fieldResolvers continue to work with extensions (by upgrading the
dependency and fixing a logic bug).
The custom fieldResolvers fix means that we now unconditionally put the
extension stack on the GraphQL context, which means that the context can no
longer be (say) a string. I changed a test that expected string contexts to
work. You couldn't use a string for a context when using extensions before, so
this isn't that big of a change.
