Explore: query transactions

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

public/app/features/explore/Table.tsx

@@ -51,7 +51,7 @@ export default class Table extends PureComponent<TableProps> {
         minRows={0}
         noDataText={noDataText}
         resolveData={data => prepareRows(data, columnNames)}
-        showPagination={data}
+        showPagination={Boolean(data)}
       />
     );
   }