JIRA: https://jira.spring.io/browse/INT-4568 * Refactor `AbstractMessageHandler` and implement `MongodbReactiveMessageHandler` * Rename `AbstractBaseMessageHandler` to `MessageHandlerSupport` * Clean up code style and resolve possible Sonar smells
JIRA: https://jira.spring.io/browse/INT-4568 * Refactor `AbstractMessageHandler` and implement `MongodbReactiveMessageHandler` * Rename `AbstractBaseMessageHandler` to `MessageHandlerSupport` * Clean up code style and resolve possible Sonar smells