Make computeScanRangeLocations and computeStats idempotent in HdfsScanNode.java

After IMPALA-12631, HdfsScanNode.computeScanRangeLocations() need check if countStarSlot_ is null or not to schedule footer range only. HdfsScanNode.computeScanRangeLocations() is called along with HdfsScanNode.init() call.

An external frontend that has its own count star slot analysis and initialization will need to recompute scan range assignment and stats after HdfsScanNode.init(). Therefore, computeScanRangeLocations() and computeStats() should be made idempotent after init() and exposed to subclasses.