Create a method in x/nftmarket which aims to define the module account to accumulate the commission fees

[taiki1frsh]

The purpose of this is simply to separate the way to send commission fee and to whom of that.
Ultimately, we're considering to hold a specific module to manage the commission fees and ways to distribute fees for the subjects.
This method works for achieving that module structure.

It can be named like RegisterCommissionPool.

[taiki1frsh]

same as this issue: #218 (comment)

[taiki1frsh]

The direction to tackle this was changed to the way we just define a const value in x/nftmarket.