Make signature batching use specialized methods

It was a mistake to use the `*_verify` methods for signature batching.
This pr move the signature batching into their own functions. This is
required, because otherwise transaction signature verification infers
with other signature verifications.

This pr also temporarily disables signature batching. The functionality
stays, but we need to make sure that all nodes have the new runtime
interface, before we can bring back signature batching.