Use cache for http.client in tsdb package. (#6833)

* datasource: move caching closer to datasource struct

* tsdb: use cached version of datasource http transport

closes #6825

pkg/tsdb/executor.go

@@ -1,6 +1,11 @@
 package tsdb
 
-import "context"
+import (
+	"context"
+	"fmt"
+
+	"github.com/grafana/grafana/pkg/models"
+)
 
 type Executor interface {
 	Execute(ctx context.Context, queries QuerySlice, query *QueryContext) *BatchResult
@@ -8,17 +13,22 @@ type Executor interface {
 
 var registry map[string]GetExecutorFn
 
-type GetExecutorFn func(dsInfo *DataSourceInfo) Executor
+type GetExecutorFn func(dsInfo *models.DataSource) (Executor, error)
 
 func init() {
 	registry = make(map[string]GetExecutorFn)
 }
 
-func getExecutorFor(dsInfo *DataSourceInfo) Executor {
-	if fn, exists := registry[dsInfo.PluginId]; exists {
-		return fn(dsInfo)
+func getExecutorFor(dsInfo *models.DataSource) (Executor, error) {
+	if fn, exists := registry[dsInfo.Type]; exists {
+		executor, err := fn(dsInfo)
+		if err != nil {
+			return nil, err
+		}
+
+		return executor, nil
 	}
-	return nil
+	return nil, fmt.Errorf("Could not find executor for data source type: %s", dsInfo.Type)
 }
 
 func RegisterExecutor(pluginId string, fn GetExecutorFn) {