RegionInfo.isOffline is now a duplication with RegionInfo.isSplit

The only place, where we set it to true is in splitRegion, and at the same time we will set split to true.

So in general, I suggest that we deprecated isOffline and isSplitParent in RegionInfo, only leave the isSplit method. And in RegionInfoBuilder, we deprecated setOffline and only leave the setSplit method.

This could make our code base cleaner.

And for serialization compatibility, we'd better still keep the split and offline fields in the actual RegionInfo datastructure for a while.