LSP Rewrite Step 2: refactor for @graphql-tools

[acao]

this gets us most of the way, just one integration suite assertion failing and some ts bugs. it needs a lot more work to be ready. the preceeding PR happens first of course

one of the end goals of this PR is also network schema invalidation :)) yes that's right, code first for all!

this also sets us up nicely for adding multi-root workspaces support, as we are reducing CPU and memory usage heavily in this PR, and this also provides a cleaner interface for MessageProcessor to support it

TODO:

• use LRU cache to lazily invalidate remote schema cache, and rewrite cached file
• create a custom "virtual file" loader to handle file changes that aren't reflected in the filesystem

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