Allow optimistic locking

[Mairu]

Sequelize with version: true option for models does internal optimistic locking. But if you want to provide your own version, to do "manual" optimistic locking from an api level you have to call instance.setDataValue('version', versionFromInput) before saving.

Would it be ok to add a optimisticLocking option to the service which does that?

I am willing to provide a pull request if it will be merged.

[daffl]

Sounds good to me as long as it can be turned off with a flag.