v0.4.0

• issue #18: Create MessageTrait with base methods for all mixins/traits.
• issue #19: [Field,FieldBuilder,Schema] Allow mixin fields to be declared overridable.
• Eliminated use of CorrelatorTrait and MicrotimeTrait as it doesn't align with the compiler strategy.
• issue #20: Add retries field to commands and request mixins.
• Moved generateMessageRef and getUriTemplateVars down to Message interface.