Refactor LSP to enable remote schema invalidation

Refactor in this case is referring to the highest level interface of the LSP server - the LSP protocol server itself. Perhaps we will make sure `runServer` is still 100% in parity as well. But `MessageProcessor` and anything inside the `GraphQLLanguageService` and `GraphQLCache` could change completely.

The `MessageProcessor` class has grown unwieldy, taking on responsibilities beyond it's duty that would make more sense for `GraphQLCache` to own, and in some cases even duplicating the file cache to some degree.

- [ ] All methods related to cacheing code-first schemas need to be migrated from `MessageProcessor` to `GraphQLCache` (so that it can make this part of the schema cache lifecycle, thus more up-to-date generated schema files!)
- [ ] All methods related to loading config and cacheing files should also be migrated from `MessageProcessor` to `GraphQLCache` - it makes more sense for the cache class to own this lifecycle, and it will make it easier to handle cache invalidation across code first and schema first scenarios.
- [ ] The `_textDocumentCache` in `MessageProcessor` should be migrated to/merged with `GraphQLCache`'s file cache

Before we begin this we refactor, we will wait for #3521 to be merged, so that we have a high quality test plane from which to detect regressions.

Then, we can fix the code first schema invalidation scenario!