Add fastify.isUnderPressure()

[mcollina]

This PR adds a decorated function to check if the current application is under pressure. This will allow applications to skip complex computations in that case (and possibly return an error).

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 the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

LGTM