Add "skip validator" mode of operation

We were recently experiencing some performance problems with the testnet node and were trying to get some more insights by starting a node locally with a snapshot of the testnet database.

However, the local validator obviously can't sign new blocks properly, so block production is failing when verifying the signature.

Still, this mode of operation might be _very_ useful from time to time because it allows for much faster iteration.

This mode of operation could then be extended to include more proof skipping which I think is also desirable for other use cases.