public final RoutingPolicy getRoutingPolicy(Utf8String protocolName, String policyName, String policyParam) {
        return getRoutingPolicy(protocolName.toString(), policyName, policyParam);
    }
Creates and returns a routing policy that matches the given arguments. If a routing policy has been created
previously using the exact same parameters, this method will returned that cached instance instead of creating
another. Not that when you replace a protocol using {@link #putProtocol(Protocol)} the policy cache is cleared.

@param protocolName The name of the protocol whose routing policy to create.
@param policyName   The name of the routing policy to create.
@param policyParam  The parameter to pass to the routing policy constructor.
@return The created routing policy.
messagebus/src/main/java/com/yahoo/messagebus/ProtocolRepository.java