refactor: Use Channels instead of Queues for packet sending, spawning, events

[andreionoie]

Also generally related to #385 and race safety, we should simply default to using System.Threading.Channels whenever we need to defer work:

• EventSystem.PendingEvents
• Connection._packetsToSend
• Map._pendingRemovals
• Map._pendingSpawns
• (in the future, if we fix bug: Thread-safety issues between packet handlers and entity state #385) PendingIncomingPacketCommands