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/core/utils/explore.test.ts

@@ -8,23 +8,18 @@ const DEFAULT_EXPLORE_STATE: ExploreState = {
   datasourceMissing: false,
   datasourceName: '',
   exploreDatasources: [],
-  graphResult: null,
+  graphRange: DEFAULT_RANGE,
   history: [],
-  latency: 0,
-  loading: false,
-  logsResult: null,
   queries: [],
-  queryErrors: [],
   queryHints: [],
+  queryTransactions: [],
   range: DEFAULT_RANGE,
-  requestOptions: null,
   showingGraph: true,
   showingLogs: true,
   showingTable: true,
   supportsGraph: null,
   supportsLogs: null,
   supportsTable: null,
-  tableResult: null,
 };
 
 describe('state functions', () => {