Fix cyclic serialization bug in RollupStats

Null Frame _fr field in completed MRTask2's so that such objects can be
Iced without passing around the Frame, which points to some Vecs which
point to RollupStats
Missing API in QuantilesPage

NOTE: this & the Quantiles.java should be folded into one file in
hex.Quantiles
Fix temp bugs & debugging prints in DRF2