Kernel: Retire SchedulerData and add Thread lookup table

This allows us to get rid of the thread lists in SchedulerData.
Also, instead of iterating over all threads to find a thread by id,
just use a lookup table. In the rare case of having to iterate over
all threads, just iterate the lookup table.

Kernel/init.cpp

@@ -162,6 +162,7 @@ extern "C" [[noreturn]] void init()
     }
     VirtualConsole::switch_to(0);
 
+    Thread::initialize();
     Process::initialize();
     Scheduler::initialize();