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/http_server.go

@@ -175,7 +175,7 @@ func (hs *HttpServer) newMacaron() *macaron.Macaron {
 	m.Use(hs.healthHandler)
 	m.Use(hs.metricsEndpoint)
 	m.Use(middleware.GetContextHandler())
-	m.Use(middleware.Sessioner(&setting.SessionOptions))
+	m.Use(middleware.Sessioner(&setting.SessionOptions, setting.SessionConnMaxLifetime))
 	m.Use(middleware.OrgRedirect())
 
 	// needs to be after context handler