BackendSrv: Queues data source requests but passes through api requests (#26947)

* Refactor: initial commit

* wip

* Refactor: getting into a simpler model

* Refactor: adds some comments

* Refactor: renames statuses according to PR comments

* Refactor: adds more comments

* Tests: adds tests for FetchQueue

* Tests: adds tests for ResponseQueue

* Tests: adds tests for FetchQueueWorker

* Tests: simplified the tests for ResponseQueue

* Refactor: adds http2 scenario

* Refactor: using Cfg instead of global variable

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Refactor: reverted change in frontendsettings.go

* Tests: fix test mocks

* Fix: changes how cfg.Protocol gets its value

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/api/frontendsettings.go

@@ -226,6 +226,7 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *models.ReqContext) (map[string]i
 		},
 		"featureToggles":    hs.Cfg.FeatureToggles,
 		"rendererAvailable": hs.RenderService.IsAvailable(),
+		"http2Enabled":      hs.Cfg.Protocol == setting.HTTP2,
 	}
 
 	return jsonObj, nil