STM: Be more optimistic when validating in-flight transactions.

* Don't lock tvars when performing non-committal validation.
* If we encounter a locked tvar don't consider it a failure.

This means in-flight validation will only fail if committing at the
moment of validation is *guaranteed* to fail.

This prevents in-flight validation from failing spuriously if it happens in
parallel on multiple threads or parallel to thread comitting.