Made GC transit feature more restrictive.

The new version is simpler, more restrictive, and better suited to its
purpose.

Also, transit objects in grace_thread_create are properly handled:
instead of being kept in transit for the duration of the thread, they
are moved to the thread specific GC stack.
Made post and poll data transit in the GC.

Also fixed ordering of "arrive" and "newslot" calls to ensure that
useful data is not GCed by another thread in between the "arrive" and
"newslot" calls.