Fix packets.go:36: unexpected EOF

Mysql has a setting called wait_timeout, which defines the duration
after which a connection may not be used anymore. Gotify doesn't
apply this, and expects the connection to work without timeout.

The fix is to set SetConnMaxLifetime, this however, isn't the
exact counterpart for wait_timeout on mysql. wait_timeout is
relative to the last use of the connection. The go setting
uses the creation of the connection as base.

Example error output:
```
[mysql] 2020/05/31 17:53:02 packets.go:36: unexpected EOF
[GIN] 2020/05/31 - 17:53:02 | 500 |     247.062µs |        10.2.2.1 | GET      "/application"
Error #1: an error occured while authenticating user

(/proj/database/client.go:24)
[2020-05-31 17:53:02]  invalid connection
```

database/database.go

@@ -3,6 +3,7 @@ package database
 import (
 	"os"
 	"path/filepath"
+	"time"
 
 	"github.com/gotify/server/v2/auth/password"
 	"github.com/gotify/server/v2/model"
@@ -35,6 +36,14 @@ func New(dialect, connection, defaultUser, defaultPass string, strength int, cre
 		db.DB().SetMaxOpenConns(1)
 	}
 
+	if dialect == "mysql" {
+		// Mysql has a setting called wait_timeout, which defines the duration
+		// after which a connection may not be used anymore.
+		// The default for this setting on mariadb is 10 minutes.
+		// See https://github.com/docker-library/mariadb/issues/113
+		db.DB().SetConnMaxLifetime(9 * time.Minute)
+	}
+
 	if err := db.AutoMigrate(new(model.User), new(model.Application), new(model.Message), new(model.Client), new(model.PluginConf)).Error; err != nil {
 		return nil, err
 	}