Harden Jamulus server for unwanted connections

Even though #2935 is already old, I think we should still consider hardening the "connection" process.
@dtinth proposed a "waiting room" approach, which we kind of already have (we already have something like this: https://github.com/jamulussoftware/jamulus/blob/2505a3154aa69459d5bcb3ae0ff8d189fdb0cb83/src/channel.cpp#L39).

> I have another suggestion for a fix.

> Problems with checking for valid audio packet is that we need to know 2 things in order to successfully decode a packet: (1) which decoder to use and (2) whether sequence numbers are used. These information is sent out-of-band and not in Opus audio packet, so we need to know their `NETW_TRANSPORT_PROPS` before we can succesfully decode an audio packet. We can also brute-force all combinations but efficiency is in question.

> So here’s a new proposal:

> Upon server receiving a packet from an unknown source, put them in a **waiting room** (do not assign a channel ID yet, dropping all audio packets in the meantime) and send it `PROTMESSID_REQ_NETW_TRANSPORT_PROPS`. It turns out that a Jamulus client can respond to that message before it has a channel ID assigned. Once an acknowledgement (or a reply, or a well-formed reply) is received, move them from waiting room to **jamming room** (where all messages are processed as usual).

_Originally posted by @dtinth in https://github.com/jamulussoftware/jamulus/discussions/2935#discussioncomment-4191832_

Beginne mit der verknüpften Logik in src/channel.cpp und verfolge dann, wie unbekannte Quellen Kanal-IDs erhalten und wie PROTMESSID_REQ_NETW_TRANSPORT_PROPS behandelt wird. Ermittle den bestehenden Ablauf des Verbindungsstatus, bevor du das vorgeschlagene Verhalten des Wartebereichs bewertest. Als abgeschlossen gilt die Aufgabe, wenn unerwünschten Quellen kein Kanal zugewiesen wird und sie keine Audiodaten verarbeiten dürfen, bis ihre Transporteigenschaften bestätigt wurden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.