Add parser and validation cache

[hugo-vrijswijk]

Split QueryCompiler.compile into prepare (parsing and document-level validation, depends only on document text and schema) and compilePrepared (variable coercion, directive validation, elaboration, depends on the per-request Env).

Add CachingQueryCompiler and QueryCache, which cache the prepare result by document text, including parse failures. Default: 1024 documents, one-hour TTL, LRU eviction. Both are configurable, and callers can supply their own QueryCache.

Add docs for setup and configuration.

[milessabin]

I'm happy with the general idea, but I think the implementation here is too specific. Maybe move the concrete implementation out of core? It should be possible to back the cache with caffeine on the JVM, for instance, or Redis.

Also, using the raw document text as a cache key is problematic IME. Normalizing it first (ie. running it through the minimzer) and hashing is a better bet.

But I think this all belongs in separate, or even external modules ... there are many different choices you might make.

[hugo-vrijswijk]

Good points, I initially leaned towards a serializable format too. But because of all the references to the schema and validations. The validation results could be dropped, but that is quite a large part of what is cached in this PR. Actual pluggable remote caches would be a big win.

For normalization, I agree this would be great. But the (current) QueryMinimizer first parses the document to its AST, and then minimizes from the AST. Which kind of defeats the point of the cache if every document still has to be parsed first. I can't really see a why to normalize without parsing. Dropping all whitespace would cause conflicts in different documents that get the same normalized output. For example, query { a b } would get the same cache hit as query { ab } while being very different documents.

I think different cache hits on different whitespace is fine. The cost is just a single reparse per document, and most applications will be built to send the same documents in most cases.

What we could do instead is only cache the parsed AST result as long as it is a Success/Warning/Failure (InternalError would need to serialize Throwable), on the (hash of) the document string, and don't cache any validation. If we provide a io.circe.Codec[Result[Ast.Document]] that would make remote caches easy to plug in to.

For a separate module, do you mean the in-memory implementation to be separate? Or the caching interface? Something like grackle-cache?