Gstreamer backend seems to leak memory

[Here](https://gist.github.com/albertz/4b8874a5f9aa9bce88180871b5aef8f6) is a small script to demonstrate the issue. The memory consumption constantly grows (up to 8GB).
See [here](https://github.com/librosa/librosa/issues/681) for a discussion.

Run the small reproducer linked in the issue and confirm the memory growth in the GStreamer backend. Read the linked librosa discussion for prior investigation, then trace the backend's resource handling; done means the reproducer no longer shows continuously increasing memory consumption.

Written by the indexing model from the issue text.