Kernel: Lock thread list while in Thread::unref()

This patch does three things:

- Convert the global thread list from a HashMap to an IntrusiveList
- Combine the thread list and its lock into a SpinLockProtectedValue
- Customize Thread::unref() so it locks the list while unreffing

This closes the same race window for Thread as @sin-ack's recent changes
did for Process.

Note that the HashMap->IntrusiveList conversion means that we lose O(1)
lookups, but the majority of clients of this list are doing traversal,
not lookup. Once we have an intrusive hashing solution, we should port
this to use that, but for now, this gets rid of heap allocations during
a sensitive time.

Kernel/init.cpp

@@ -207,7 +207,6 @@ extern "C" [[noreturn]] UNMAP_AFTER_INIT void init(BootInfo const& boot_info)
     __stack_chk_guard = get_fast_random<size_t>();
 
     ProcFSComponentRegistry::initialize();
-    Thread::initialize();
     Process::initialize();
 
     Scheduler::initialize();