go batch: marshaling index as a number

[cdauphinee]

The index struct is currently marshaling to/from "{}", which makes it hard to add diagnostic logging to code that uses it.

This change marshals the index using its internal key; for example:

index1 := batch.NewIndex(1)
index2 := batch.NewIndex(2)
data := map[batch.Index]string {
  index1: "foo",
  index2: "bar",
}

json, _ := json.Marshal(data)

// json before change
{
  "{}": "foo",
  "{}": "bar"
}

// json after change
{
  1: "foo",
  2: "bar"
}