Distinction between testnet and mainnet assets

[ahtotruu]

Original proposal from @vrogojin:
https://github.com/unicitynetwork/unicity-yellowpaper-tex/blob/network-id-realms/network-id-realms-proposal.md

In the first iteration, we implement a more limited scope with static realms and one network per realm, meaning we do not need to distinguish between realms and networks for now. Hence the sub-issues below should be sufficient.