(gc_section_count): Removed, thread-sleeping can not

(gc_section_count): Removed, thread-sleeping can not
be nested.
(scm_i_thread_put_to_sleep): Call scm_i_leave_guile before locking
admin mutex so that we can be put to sleep by other threads while
blocking on that mutex.  Lock all the heap mutex of all threads,
including ourselves.
(scm_i_thread_wake_up): Unlock all threads, including ourselves,
call scm_i_enter_guile.
(scm_thread_mark_stacks): Expect all threads to be suspended.