Avoid possible class loading deadlock with VerifierNone initialization

Superclass Verifier has a static initializer VERIFIER_NONE that initializes sub-class VerifierNone. This reference can result in deadlock during class loading as per https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.4.2.

As of today, only RpcProgram use this instance and hence it is safe but if more clients start using this (specifically static ones), it has potential to bring deadlock. We should break this referencing before it is late.