Minersc payFes: fix sharder delegate reward and view change interest

[Sriep]

Fix a couple of issues in fees.go.

• The service change for sharder delegates was wrong. The service charge was not being removed. This was also fixed in PR Fixing rewards and fees + fees_test, lots of refactoring and other small fixes #117 but that came with extensive refactoring. This aims to change as little as possible.
• Interest payments not being paid. Now interest payments are paid every multiple of sc.minersc.reward_round_frequency, this is the same irrespective of the view_change setting. There is an alternative approach of paying interest when 0chian.view_change = true which will mostly work out the same.

Resolves issue indicated by unit test PR #137 which cannot be merged until this PR is merged.