Fix duplicate label check to work with selector batch size

[yeya24]

Create this issue for an existing TODO in the code https://github.com/thanos-io/promql-engine/blob/main/execution/exchange/duplicate_label.go#L62.

	// TODO: currently there is a bug, we need to reset 'd.c's state
	// if the current timestamp changes. With configured BatchSize we
	// dont see all samples for a timestamp in the same batch, but this
	// logic relies on that.

It looks like there is a bug in duplicate label check operator when working with selector batch. We need to fix it.