Improve Spark data quality

[bajtos]

Form majorities in each committee and cross-check that measurement fields have the same values in the majority. Consider dropping measurements from committees that are too small to give us enough confidence that the majority is honest.

Related:

• Find majorities in Spark measurements #59
• Robust fraud detection spark-checker#22
• Stable committee size spark-checker#32
• Deterministic task<->node assignment spark-checker#38