It's possible and normal to tear down a harness while the pipeline is
running. At the same time, it's desired for the
`gst_harness_pad_link_tear_down()` function to be synchronous.
This has created the conflict where the main thread may request a
harness to be torn down while it's in use or about to be used by a pad
in the streaming thread.
The previous implementation of `gst_harness_pad_link_tear_down()` tried
to handle this by taking the stream lock of the harnessed pad and
resetting all the pad functions while holding it. That approach was
however insufficient to handle the case where a non-serialized event
or query is being handled or about to be handled in a different thread.
This edge case was one race condition behind the flakes in the flvmux
check tests -- the rest being covered by https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2803.
This patch fixes the problem by adding an intermediate ref-counted
object, GstHarnessLink, which replaces the usage of the HARNESS_KEY
association. GstHarnessLink allows the pad functions such as event,
query and chain to borrow a reference to GstHarness and more
importantly, to lock the GstHarnessLink during their usage to block
(delay) its destruction until no users are left, and guarantee that any
future user will not receive an invalid GstHarness handle past its
destruction.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5017>
