graphql: cache GraphQL validation

[kamilkisiela]

I ran the same query ~55 times and got these results:

No caching (58 samples)

p95:      47.75µs
p99:      73.83µs
average:  24.85µs
median:   20.13µs

Caching (56 samples)
    
p95:       4.63µs
p99:       4.92µs
average:   2.08µs
median:    1.67µs

Part of #3077

[kamilkisiela]

Applied suggested changes, ran cargo fmt and squashed commits

[leoyvens]

Still doesn't build.

[leoyvens]

Lock lifetimes are tricky. I've built on your changes so that the lock is only held when reading or inserting to the cache, but importantly not held when actually validating the query. As you had noticed this requires a bit of cloning.

[leoyvens]

@kamilkisiela could you review and re-run the benchmarks?

[kamilkisiela]

@leoyvens thank you. I re-run the benchmarks and it looks faster now.