Cleanup unnecessary usages of MetaTableAccessor

For simplify the implementation of splittable meta.

In general, MetaTableAccessor can not deal with the catalog information for meta itself, even if we introduce a general root table, we could make it also deal with the catalog information for meta, but then we still need to deal with root...

Let's change to use other APIs if possible, for example, when getting all region locations, just use RegionLocator instead of MetaTableAccessor.