aens_preclaim_optional

• Make NamePreclaimTx optional. Since the introduction of auctions, the front-running protection offered by the 'PreClaim -> Claim' flow is no longer as important. To simplify name registrations (or the start of a name auction) we now allow NameClaimTx without a previous NamePreclaimTx. In this case we set the NameSalt to 0.