Subtask of #68 * [x] Add `ConversationManager` method for deleting all conversation messages which delegates to conversation clients * [x] Add `ConversationClient` method for deleting all messages that returns true if all messages were delete or false if not (because they are invitation/introduction messages that can't be deleted). * [x] Add real implementation for `MessagingManager`, no-op implementations for other clients (just return false)