skins leak between sessions (SkinTracker not cleared on disconnect)

[duzos]

split off from #26 - the skin tracker has the exact same leak.

SkinTracker.initClient registers a ClientPlayConnectionEvents.DISCONNECT handler but the clear inside it is commented out (SkinTracker.java:67-69):

ClientPlayConnectionEvents.DISCONNECT.register((handler, client) -> {
    // getInstance().clear();
});

SkinTracker is a HashMap<UUID, SkinData> that's filled from the server on sync (receive, SkinTracker.java:131-139 - just puts, never clears first). so when you leave a server its skins stay in the map and bleed into your next world / server.

fix: uncomment the getInstance().clear(); on DISCONNECT. could also clear at the start of receive for a clean resync.

same root cause as #26, and #21 (native registries) would kill this whole class of leak for free too.