transport: Don't hardcode a specific raw audio format

There's nothing wrong with using another one and it might safe us from doing
some conversions.
transport: Use atomic integer ops for the unique payload counter
payload: Clean up the payload's "media-type" property and make it an abstract type

The "media-type" property is really a read-only property that is defined by
the subclasses... so implement it as such.
local: Free list and all capture sources after notifying the callback about them
transport: Unref rtpbin session after usage
transport: Unset GValues after g_closure_invoke()
transport: Use the generic cclosure marshaller