Avoid blocking resource release by client code
Avoid blocking resource release by client code

The change in #1520 unnecessarily waits for a client-provided callback
to complete before release resources that are no longer necessary.
Won't have any impact in microbenchmarks that have no-op callbacks.
Release as early as possible.

pion/webrtc,pion/webrtc,pion/webrtc,pion/webrtc