This adopts the full networking stack from lighthouse

Code is adopted from lighthouse, which is licensed under Apache-2. Unfortunately
because the dependencies we use are different, it's not possible to directly
depend on upstream.

So that we're on a version that supports gossipsub, thanks to lighthouse.

This copies blockchain-network-libp2p crate as the foundation to continue to
build upon.

* Move blockchain::rocksdb into a separate crate

* Add license and description for blockchain-rocksdb