Standalone relay decision service - stage 2 - Implement standalone relayer decision strategy

[cam-schultz]

Context and scope
Umbrella issue: #362

Discussion and alternatives

• If the sidecar is unavailable/disabled, the main relayer application should support the following strategies:
  • Default to true
  • Default to false
  • Emit a fatal error
  • Relay based on "correctness" - i.e. existing ShouldSendMessage implementation
• Existing ShouldSendMessage implementation should be lifted into a common package that can be consumed by the main relayer application as well as the Decider

Open questions
How much decision logic should the main relayer application implement directly? See the discussion in the umbrella issue #362