CIS-7/Add distinction in representation of cis7 accounts and tokenholder

[soerenbf]

Ref COR-1865

Purpose

Factor AccountAddress of CIS7 out of TokenHolder.

Changes

• TokenHolder represents the corresponding protobuf type
• CborAccountAddress represents the CIS7 account address.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.