If in server config mysql has autocommit = 0, then wikijs fails with locks like this:
```
2020-10-30T12:56:51.725Z [JOB] error: Rebuilding page tree: [ FAILED ]
2020-10-30T12:56:51.726Z [JOB] error: truncate `pageTree` - Lock wait timeout exceeded; try restarting transaction
```

+ Remove duplicated await
+ Replace some legacy codes with ES6
+ Fix some of eslint problems