Validation promise performance

[DaddyWarbucks]

When using large, deeply nested schemas with lots of sync tests and only one async test, calling schema.validate() converts ALL tests to promises. This means that a schema with 100 sync validation tests (or nested tests) and one async validation test will use 101 promises to validate the schema.

I first noticed this issue when validating an array of 5,000 items with a rather large nested schema but only one async validation that was actually a noop. The validation would take up to 10 seconds. But when removing that validation and calling validateSync the time dropped to roughly 2 seconds.

The included PR updates the createValidation function in a manner that only pushes validations onto the event loop if they are a promise, otherwise it validates as a sync function. This avoids scheduling sync work in the event loop and leads to some major performance improvements.