EventLoss analyzer for detecting event loss

[mayurdb]

Loss of critical events like StageEnd, JobEnd, ExecutorAdded, etc leads to inaccurate reports. https://github.com/qubole/sparklens/issues/56 highlights this problem. We should write an EventLossDetector Analyzer which will detect the event loss and communicate to the user that the report might not be accurate or not show the report altogether.