session: fork Macaron mysql session middleware

This changes forks the mysql part of the Macaron session middleware.

In the forked mysql file:

- takes in a config setting for SetConnMaxLifetime (this solves wait_timeout
problem if it is set to a shorter interval than wait_timeout)
- removes the panic when an error is returned in the Exist function.
- retries the exist query once
- retries the GC query once

pkg/api/common_test.go

@@ -99,7 +99,7 @@ func setupScenarioContext(url string) *scenarioContext {
 	}))
 
 	sc.m.Use(middleware.GetContextHandler())
-	sc.m.Use(middleware.Sessioner(&session.Options{}))
+	sc.m.Use(middleware.Sessioner(&session.Options{}, 0))
 
 	return sc
 }