stats: Measure percentiles, max etc.

[varungandhi-src]

Fixes #239
Fixes ENG-23701

Test plan

Ran it locally, it prints the following output for a Chromium index.

{
  "documents": 103101,
  "documentSizes": {
    "Percentiles": {
      "50": 10994,
      "90": 116245,
      "95": 224756,
      "99": 747849,
      "99.9": 2806214
    },
    "mean": 57509,
    "stddev": 287100,
    "max": 43255012,
    "sum": 2147483647,
    "comment": "sizes are in bytes"
  },
  "linesOfCode": 0,
  "occurrences": 80690299,
  "occurrenceCounts": {
    "Percentiles": {
      "50": 145,
      "90": 1611,
      "95": 3043,
      "99": 10044,
      "99.9": 38139
    },
    "mean": 782,
    "stddev": 3985,
    "max": 589825,
    "sum": 80690299,
    "comment": ""
  },
  "definitions": 10405028,
  "definitionCounts": {
    "Percentiles": {
      "50": 31,
      "90": 206,
      "95": 361,
      "99": 1095,
      "99.9": 3966
    },
    "mean": 100,
    "stddev": 395,
    "max": 43055,
    "sum": 10405028,
    "comment": "counted using occurrences"
  }
}