Abandon small numbers of straggler reports with funky timestamps instead of creating small aggregation jobs

[tgeoghegan]

Reports that arrive very late (or appear to arrive very late due to bad client clocks) can cause Janus to create very small aggregation jobs that can then pathologically delay the running of collection jobs. We could change our job creation/readiness policy to allow abandoning small numbers of reports in the name of keeping throughput up.