MTU Value is shared across all peers

[austin-space]

Description

MTU values are shared across all connected peers, meaning that if the value is negotiated up by the transport, it still can't be upgraded because it would get bumped up for clients that have not yet negotiated the value up.

Reproduce Steps

1. Create a unity dedicated server using NFGO and a custom transport that does mtu negotiation.
2. When MTU is negotiated up successfully for any single client, set MaxTransmissionUnitSize to the new value.
3. Observe that this changes the mtu for all clients.

Actual Outcome

The MTU is changed for all clients.

Expected Outcome

The MTU should only be changed for the single client in question.

Environment

• OS: Windows
• Unity Version: 2020.3
• Netcode Version: 1.5.1

Additional Context

This is really just a rehashing of #2431 which was closed prematurely. The solution(#2530) is technically correct as long as there is only one connection, which means that it works for clients, but not for servers.