Prevent spamming with `register` and `email` transactions

[thantsintoe]

In the current network, a malicious actor can inject lots of register and email transactions without any restriction. There should be a way to prevent it.

[thantsintoe]

@osyed can you advise on this issue?

[thantsintoe]

Will the alias register tx require the user to pay txFee?

Not for the initial username mapping when the account is created. But we can require some PoW to be provided with this to prevent abuse.