[RFC] Cache parsed and validated queries in memory

[mxstbr]

A first RFC to kick off a discussion about caching parsed and validated
queries. This is based on the ideas from graphql/graphql-js#158

GraphQL APIs often get the same queries with different variables over
and over. Even though the query string doesn't change, Apollo server
parses and validates it on every request.

Instead, we can cache the resulting AST and skip the parsing and
validation step on further requests with the same query. While I don't
think this will provide an order of magnitude of a speedup, it seems
like it would avoid unnecessary work, could help in performance
sensitive scenarios and doesn't add much complexity.

This PR implements this in a very crude way to kick off a discussion
about including this in Apollo Server.

TODO:

• Decide on scope (external caches? persisted queries?)
• Add an option to disable/enable/configure
• Make it a LRU cache to avoid balooning memory?
• TBD

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[martijnwalraven]

@mxstbr Thanks for looking into this! But there is actually an existing PR for this in flight already: #2111

[mxstbr]

Nevermind I'm a dummy! Will follow #2111!