Validate static schema on middleware mount

[ValeriaVG]

Problem:
Static schema only gets validated on the first request, which causes server to successfully start and pass health checks

Solution:
If static schema is provided - validate it on mount and throw an error if validation fails

[IvanGoncharov]

@ValeriaVG Totally agree that it should be done 👍
One issue that I see is that it would be extremely hard to debug schema validation errors.
As a general rule schema should be validated once in user code, we plan to add something like GraphQLFinalizedSchema (validated and frozen) in graphql-js and force users to call something like schema.finalize() in their code.

I will keep this PR as a bug report until it is implemented in the next version of express-graphql.