fix(schema): Performance improvements for better AST caching

[klausi]

Fixes #1312, reworked from #1314 .

[Kingdutch]

I think in general this is a step forwards from where we are. With 417c308 I'd like to go a bit further that includes a few differences:

• Preservation of the source locations allows for better debugging
• Moving the separation of the base schema and the schema extension document into a single function provides more power to extending modules such as graphql_compose and graphql_core_schema which usually don't have a separation between a base schema or extensions. People who don't overwrite schema generation will usually use the default file loading methods anyway.
• Schema printing itself is quite slow for the uncached case, an in-memory AST transformation is faster

Unfortunately the referenced commit is blocked by the need for a utility in graphql-php which is slightly smarter than what's included now.

My only request for this PR is to mark the two new methods buildSchema and getFullSchemaDocument as private. That way we can still jump to my proposed commit in 4.x without having to break everyone's code. If we keep them as protected then people might overwrite them and I no longer see how I'd get from this PR to the code in my commit without risking the breakage of users of the GraphQL module.

[klausi]

Agreed, pushed a commit to make them private.