Introduce parallel send_message API

[cBournhonesque]

Update the Message/Trigger API:

• previously we had access to ConnectionManager.send_message/send_trigger, which had the benefit of only requiring a reference to the message, but limited parallelism
• this PR adds another way (inspired by replicon) to send message: you can just buffer them to Events<ClientSendMessage<M>> or Events<ServerSendMessage<M>>, or call commands.client_trigger() or commands.server_trigger(), however you need to provide an owned version of the message

I had some trouble with the API so I removed the ConnectionManager.send_trigger event.

This PR also renames some things:
-> MessageEvent becomes ClientMessageEvent or ServerMessageEvent
-> app.register_event() becomes app.register_trigger(), and is specifically for replicated triggers, not messages