v012: Starname module: set a default price at open domain

[dsmello]

• Enable the owner of the open domain to set a default price for the accounts at that domain.

[xpt1x]

Introduce tx MsgSetDefaultAdditionalAccountsPrice only applicable for open domains

Should we add a field to do this with MsgRegisterDomain as well?
IMO: Lets keep this separate for now, as MsgRegisterDomain applies to both open and closed domain registration

Client can batch RegisterDomain and SetDefaultAdditionalAccountsPrice tx based on situation