kernel: Introduce Thread::time_lock and Team::time_lock
kernel: Remove gSchedulerLock

 * Thread::scheduler_lock protects thread state, priority, etc.
 * sThreadCreationLock protects thread creation and removal and list of
   threads in team.
 * Team::signal_lock and Team::time_lock protect list of threads in team
   as well.
 * Scheduler uses its own internal locking.