Fix a race condition with KisMetaData::SchemaRegistry singleton

So I got this crash once. At the time I had three TIFF files in the recent file list. Since the recent files thumbnail icons are loaded in parallel for now, I suspect it happened to trigger a race condition when initializing the SchemaRegistry::instance() singleton.

This change makes the singleton actually thread-safe by using a static local. Making an MR so that someone can sanity check my thinking.