audio: video: Optimize by using cached quark for meta tag

added 485 commits

88314354...648acbe7 - 484 commits from branch gstreamer:master
08a34fea - audio: video: Optimize by using cached quark for meta tag

resolved all threads

So I was going to add the g_once_init_{enter,leave}() bits to the quark getters, then I noticed that the API naming is awkward (gst_meta_tag_audio_quark() rather than gst_audio_*), then I noticed that this is mostly for internal use in the libs anyway, apart from audioconvert/audioresample, and then I couldn't find users for this in good/bad/ugly/elsewhere (other than the rtp plugin that already does some manual caching), so then I started to make it an internal helper function in gstaudio/video, but in the end it seemed easier if every class just does its own private cache for the quark. Better in terms of performance because we can init it in _class_init() and don't need locking or atomic ops, and probably better in terms of code size + binary size too. So I went for KISS for now. We can always decide to expose API for it later after all if we want.