fix: #1353 ignore evaluation of $schema field in json-schema

[Eomm]

Hi,
this is the fix for #1353 issue, where $schema field is ignored and it is not resolved.
I checked that there aren't other fields like that, and I didn't notice them.

I have run also benchmark (with some problems 😅), but looking the code I think that this change impact only the startup time right?
Do you need this fix starting from branch 1.x also?

Thank you

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

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Running banchmarks I get these error after some succesfull run, but maybe could be caused by PC?

{"level":50,"time":1545772614031,"msg":"client error","pid":14584,"hostname":"EOMM-XPS","err":{"type":"Error","message":"write ECANCELED","stack":"Error: write ECANCELED\n    at _errnoException (util.js:992:11)\n    at WriteWrap.afterWrite [as oncomplete] (net.js:864:14)","code":"ECANCELED","errno":"ECANCELED","syscall":"write"},"v":1}

[cemremengu]

Thanks for the PR! Yes, the schemas are precompiled for efficiency (fastify's secret sauce) and we will need to backport this for 1.x as well

[mcollina]

LGTM

[cemremengu]

LGTM

[delvedor]

LGTM

[delvedor]

Backported to 1.x in 0ee57e7.

[Eomm]

Thank you guys 😃

[github-actions]

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