`get_avatar()` was checking the cache directly, bypassing `plugin_misc_identity`'s
`use_cache` flag. As a result, cache disabling was ineffective. It now fetches from the
PEP node and doesn't handle the cache anymore.
`_parse_metadata_info()` has been added to factorize the metadata parsing.
- Avatar disabling was not correctly handled for all cases, this should now be fixed.
- Avatar retrieval is now down only on PEP events, no directly anymore. `get_avatar` only
returns cached version.
`NS_AVATAR_METADATA` is already advertised by `add_pep_event`, adding it to `getDiscoInfo`
too was resulting in double advertising, making the disco infos invalid for caching
(resulting in other devices repeatedly requesting disco#info).
