eth: rewardservice to use round subscription

[kyriediculous]

What does this pull request do? Explain your changes. (required)
Remove polling on a set interval (which executes a ton of RPC requests) to only trying reward when a new round event is received from the TimeWatcher.

Specific updates (required)

• Start reward service with time watcher
• use a go routine to start reward service, remove the go-routine inside of rewardService.Start and block instead. This follows the convention of most of our other services
• use an interface for the TimeWatcher so that we can unit test this service
• Add basic reward service unit tests
• Add some methods to the eth.MockClient to facilitate unit tests
• Use a single channel serviceErr to catch errors from startup goroutines for several services (currently rewardService and redeemer)

How did you test each of these updates (required)
Manually tested using the devtool and added log events
Unit tests

Does this pull request close any open issues?
Fixes #1584

Checklist:

• README and other documentation updated
• Node runs in OSX and devenv
• All tests in ./test.sh pass

[yondonfu]

LGTM after squashing