Cache pre-compiled AJV validators

[bajtos]

The initial implementation of request-body validation uses the more convenient but significantly less-performant API from AJV:

https://github.com/strongloop/loopback-next/blob/34eba3498528fe70fa9e64e8582d30a5b255e35b/packages/rest/src/validation/request-body.validator.ts#L93-L103

As a result, a typical POST (create) request is 10x slower than a GET (find) request.

We should rework our validation to cache pre-compiled AJV validators, see docs.

• To avoid slow application start, validators should be precompiled lazily (on the first request).
• Precompiled validators can be stored for example as a new property of the RouteEntry class.

Acceptance criteria

• Modify request-body validator to cache pre-compiled validators
• Re-run our benchmark and update its README with the newly observed data