libs/gst/base/gstbasesrc.c: Unify flushing code, remove some old unlock code that...

libs/gst/base/gstbasesrc.c: Unify flushing code, remove some old unlock code that is no longer used.

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_default_query),
(gst_base_src_perform_seek), (gst_base_src_default_event),
(gst_base_src_set_flushing), (gst_base_src_activate_push),
(gst_base_src_activate_pull):
Unify flushing code, remove some old unlock code that is no longer used.
Take the streaming lock when seeking to avoid races. Fixes #492729.
Added some more comments.