Bakers reward period

[MilkywayPirate]

Purpose

Solve https://github.com/Concordium/concordium-rust-sdk/issues/107

Checklist

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