Add Unique Queue infrastructure and move TestPullRequests to this (#9856)

* Upgrade levelqueue to version 0.2.0

This adds functionality for Unique Queues

* Add UniqueQueue interface and functions to create them

* Add UniqueQueue implementations

* Move TestPullRequests over to use UniqueQueue

* Reduce code duplication

* Add bytefifos

* Ensure invalid types are logged

* Fix close race in PersistableChannelQueue Shutdown

go.mod

@@ -4,7 +4,7 @@ go 1.13
 
 require (
 	cloud.google.com/go v0.45.0 // indirect
-	gitea.com/lunny/levelqueue v0.1.0
+	gitea.com/lunny/levelqueue v0.2.0
 	gitea.com/macaron/binding v0.0.0-20190822013154-a5f53841ed2b
 	gitea.com/macaron/cache v0.0.0-20190822004001-a6e7fee4ee76
 	gitea.com/macaron/captcha v0.0.0-20190822015246-daa973478bae