Distribution model of trading DTag

[kwunyeung]

Feature description

Further idea development with the trading DTag mentioned in #225 . If the DTag can be traded, the trading fee should not be fully put in the DTag seller. On Desmos, we are trying to discourage the idea of "first comer takes all". Of course early adopters have a higher chance to a DTag they desire but we should not make it something like domain name trade marketing. Prices should be set free but the network should take a high tax and share the profit to the the whole network but not just a single user. For example, the network will take a 50% as community donation and will then distribute to other community contributors.