Kernel: Make Thread refcounted

Similar to Process, we need to make Thread refcounted. This will solve
problems that will appear once we schedule threads on more than one
processor. This allows us to hold onto threads without necessarily
holding the scheduler lock for the entire duration.

Kernel/Net/NetworkTask.cpp

@@ -61,7 +61,7 @@ static void handle_tcp(const IPv4Packet&, const timeval& packet_timestamp);
 
 void NetworkTask::spawn()
 {
-    Thread* thread = nullptr;
+    RefPtr<Thread> thread;
     Process::create_kernel_process(thread, "NetworkTask", NetworkTask_main);
 }