Allow nodes to sync without a checkpoint (from genesis) in networks created from a converted ZQ1 persistence

[JamesHinshelwood]

In theory, nodes should be able to download the blocks and transactions from peers - We'll probably need to add a 'hard-fork' flag to say "Don't execute these blocks and don't store any state".

We will also have to think about the best way to share the final ZQ1 state though - I haven't got any good ideas yet.