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/services/session/session.go

@@ -6,7 +6,6 @@ import (
 
 	ms "github.com/go-macaron/session"
 	_ "github.com/go-macaron/session/memcache"
-	_ "github.com/go-macaron/session/mysql"
 	_ "github.com/go-macaron/session/postgres"
 	_ "github.com/go-macaron/session/redis"
 	"github.com/grafana/grafana/pkg/log"
@@ -25,6 +24,7 @@ var sessionOptions *ms.Options
 var StartSessionGC func()
 var GetSessionCount func() int
 var sessionLogger = log.New("session")
+var sessionConnMaxLifetime int64
 
 func init() {
 	StartSessionGC = func() {
@@ -37,9 +37,10 @@ func init() {
 	}
 }
 
-func Init(options *ms.Options) {
+func Init(options *ms.Options, connMaxLifetime int64) {
 	var err error
 	sessionOptions = prepareOptions(options)
+	sessionConnMaxLifetime = connMaxLifetime
 	sessionManager, err = ms.NewManager(options.Provider, *options)
 	if err != nil {
 		panic(err)