Cache the result of validating schemas

[istreeter]

The client's check method always validates the schema against jsonschema v4 before checking the data is valid against the schema. For a high volume application, with a small number of schemas, this means we are performing the same validation over and over again. We are already using caching in this lib, so makes sense to also cache this validation result.

[istreeter]

This got implemented via #207