New PJSUA callbacks to notify application when media stream is created and destroyed

[bennylp]

New callbacks:

• on_stream_created(): this callback is called to notify application when stream is created and about to be registered to conference bridge. Application may use this callback to add media processing ports to the stream.
• on_stream_destroyed(): this callback is called to notify application when stream is about to be destroyed. If application created media processing ports to the stream, it may use this callback to clean-up resources associated with the media stream. Also if application manages it's own media (see [FAQ#pjsua-lib-perf this FAQ entry]), application will need to use this callback to signal media processing that stream is about to be destroyed.

Thanks Norman Franke and Truong Thanh Quang for the suggestions.

[bennylp]

Done in r1806. Thanks Norman Franke for the patch.