gateway constructor ordering issue

in apollo gateway constructor, the line `if(config){}` must be set before line `this.schema = this.createSchema`, otherwise this.experimental_didFailComposition will not work for the first time!

Start at the Apollo gateway constructor and inspect the ordering of the config handling and schema creation. Verify that the first composition failure is handled when experimental_didFailComposition is enabled; done means config is processed before createSchema and the first failure behaves correctly.

Written by the indexing model from the issue text.