BufferSource nodes should be cleaned up after playback

[ericrosenbaum]

Expected behavior

Each time you play a sound, a webaudio buffersource node is created. After the sound ends, the node should be garbage collected.

Actual behavior

I'm not totally sure but I think that they can not be garbage collected. We store a reference to each buffer source node in order to set the playback rate while the sound plays (for the pitch effect), so at the very least these references need to be deleted after the sound ends.

Note that we cannot debug this using the Chrome Web Audio Inspector, because its own operation prevents garbage collection of nodes (see here)

Operating system and browser

Mac OS 10.11.6 Chrome 59.0