code: function initializeUnique() {
    uniquecoll = db.addCollection('uniquecoll', {
      unique: ['customId']
    });
  
    for (var idx = 0; idx < arraySize; idx++) {
      var v1 = genRandomVal();
      var v2 = genRandomVal();
  
      uniquecoll.insert({
        customId: (arraySize - idx),
        val: v1,
        val2: v2,
        val3: "more data 1234567890"
      });
    }
  }
docs: initializeUnique : to support benchUniquePerf, we will set up another collection
  where our customId is enforced as 'unique' using unique index feature of loki.