audioaggregator: Make access to the pad list thread-safe while mixing

audioaggregator: Make access to the pad list thread-safe while mixing

When mixing every single buffer the object lock is shortly released and
acquired again. In the meantime the pad list can become invalid because
a pad was removed or added, and equally the current pad might as well
have been finalized in the meantime.

To get around that, take a snapshot of all sinkpads before mixing and
work with that list of pads.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3052

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5553>