gst/schedulers/gstoptimalscheduler.c: Added lock to protect scheduler data structures.

gst/schedulers/gstoptimalscheduler.c: Added lock to protect scheduler data structures.

Original commit message from CVS:
* gst/schedulers/gstoptimalscheduler.c:
(gst_opt_scheduler_class_init), (gst_opt_scheduler_init),
(gst_opt_scheduler_finalize), (remove_decoupled), (schedule_chain),
(get_invalid_call), (chain_invalid_call),
(get_group_schedule_function), (loop_group_schedule_function),
(gst_opt_scheduler_loop_wrapper), (gst_opt_scheduler_get_wrapper),
(gst_opt_scheduler_state_transition),
(gst_opt_scheduler_add_element),
(gst_opt_scheduler_remove_element), (gst_opt_scheduler_interrupt),
(gst_opt_scheduler_error), (gst_opt_scheduler_pad_link),
(gst_opt_scheduler_pad_unlink), (gst_opt_scheduler_iterate),
(gst_opt_scheduler_show):
Added lock to protect scheduler data structures.