Existing querying was grouped together before handed over to the
datasource. This slowed down result display to however long the slowest
query took.

- create one query transaction per result viewer (graph, table, etc.)
  and query row
- track latencies for each transaction
- show results as soon as they are being received
- loading indicator on graph and query button to indicate that queries
  are still running and that results are incomplete
- properly discard transactions when removing or changing queries