Core/Networking: Rewrite networking threading model

Each network thread has its own io_service - this means that all operations on a given socket except queueing packets run from a single thread, removing the need for locking
Sending packets now writes to a lockfree intermediate queue directly, encryption is applied in network thread if it was required at the time of sending the packet

(cherry picked from commit 97a79af470)

src/server/authserver/Server/AuthSession.cpp

@@ -274,10 +274,7 @@ void AuthSession::SendPacket(ByteBuffer& packet)
     {
         MessageBuffer buffer;
         buffer.Write(packet.contents(), packet.size());
-
-        std::unique_lock<std::mutex> guard(_writeLock);
-
-        QueuePacket(std::move(buffer), guard);
+        QueuePacket(std::move(buffer));
     }
 }