.rsk Registrar

The registrar is separated into several components for simplicity, modularity, and privilege minimization. Rootstock (RSK) Owner is the owner of rsk top level domain, so it is the only contract that can invoke setSubdomainOwner in RNS Registry. It grants access to other contracts for registering new domains and/or renewing domain's expiration time. Currently we've enabled a first-in first-served registrar contract and a simple renewer contract that enable minimal actions to provide basic functionality for domain creation and administration.