Add new `default` option for timestamp field

Index process fails when having `_timestamp` enabled and `path` option is set.
It fails with a `TimestampParsingException[failed to parse timestamp [null]]` message.

Reproduction:

```
DELETE test
PUT  test
{
    "mappings": {
        "test": {
            "_timestamp" : {
                "enabled" : "yes",
                "path" : "post_date"
            }
        }
    }
}
PUT test/test/1
{
  "foo": "bar"
}
```

You can now define a default value for when timestamp is not provided
within the index request or in the `_source` document.

By default, the default value is `now` which means the date the document was processed by the indexing chain.

You can disable that default value by setting `default` to `null`. It means that `timestamp` is mandatory:

```
{
    "tweet" : {
        "_timestamp" : {
            "enabled" : true,
            "default" : null
        }
    }
}
```

If you don't provide any timestamp value, indexation will fail.

You can also set the default value to any date respecting timestamp format:

```
{
    "tweet" : {
        "_timestamp" : {
            "enabled" : true,
            "format" : "YYYY-MM-dd",
            "default" : "1970-01-01"
        }
    }
}
```

If you don't provide any timestamp value, indexation will fail.

Closes #4718.
writeString would fail if the default timestamp is null.

So I think we would also need to write a boolean to tell whether it is not null? (and an integration test that would exercise serialization)