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/middleware/middleware_test.go

@@ -338,7 +338,7 @@ func middlewareScenario(desc string, fn scenarioFunc) {
 		sc.m.Use(GetContextHandler())
 		// mock out gc goroutine
 		session.StartSessionGC = func() {}
-		sc.m.Use(Sessioner(&ms.Options{}))
+		sc.m.Use(Sessioner(&ms.Options{}, 0))
 		sc.m.Use(OrgRedirect())
 		sc.m.Use(AddDefaultResponseHeaders())