add optimistic locking option

[hiradimir]

please see title.

---

but
package-lock.json auto updated.

I did only this command.

npm install && npm run build && npm run cover

My environment.

$ node --version
v7.5.0
$ npm --version
5.3.0

[RobinBuschmann]

@hiradimir, looks good. Thanks for adding this :)

[hiradimir]

@RobinBuschmann Thanks you for merging!