Ensure task names are unique

[danielmitterdorfer]

Consider the following schedule:

[
    {
      "operation": "my-query",
      "iterations": 100
    },
    {
      "operation": "force-merge"
    },
    {
      "operation": "my-query",
      "iterations": 100
    }
]

Rally uses the operation's name to record metrics. In that case, the summary report will mix the metrics for both invocations of my-query (before and after force-merge) and this is likely not wanted.

Rally should - by default - still use the operation's name as the task name but it should check for duplicates and raise an error. Users then need to define a new property name which defines the task's name. E.g.:

[
    {
      "name": "my-query-no-force-merge",
      "operation": "my-query",
      "iterations": 100
    },
    {
      "operation": "force-merge"
    },
    {
      "name": "my-query-after-force-merge",
      "operation": "my-query",
      "iterations": 100
    }
]

Note: Marking as breaking as tracks that previously got this wrong will start to fail at this point.

[danielmitterdorfer]

Reopening in order to also enhance the metrics format (we should store the task and operation name).