* remove redundant parameter from method call.
* remove uncessary threadpool.
* Simplify `GossipCore.sendOneWay()`
* Cleanup useage of `MessageInvoker`
* `DefaultMessageInvoker` replaced by a factory
* `MessageInvokerCombiner` replaced by same factory
* Alter `MessageInvokerTest` to not rely on specific types
* Remove type assertion from `GossipManagerBuilderTest`
* Merge `MessageInvoker` with `MessageHandler`
* This required changing method signature return type from `void` to `boolean`.