MixedDatasource: Support streaming from many different data sources

Using master 212bf7a4f6 and developing a streaming datasource, I stumbled upon this issue: In the panel's query editor, when selecting my streaming datasource globally for all queries, everything works fine : data points appear on the graph as they are received. However, when selecting `-- Mixed--` and then the right DS in the query breaks the panel. No new data is displayed and the progress icon on the top right keeps rolling.