Add rewards handling chainspec setting

[darthsiroftardis]

CHANGELOG:

• Adds a new rewards_handling mode to the chainspec

[zie1ony]

Could you please describe what this PR do (it doesn't just add a flag to chainspec)? I think, I've understood it, but it would be useful to see what was your intent.

Also, the code is not commented at all, which makes it really hard to understand. Could you please comment all public functions and add short descriptions to the test scenarios? (given/when/then format would be great).