ORM Concurrency Control

**Notice**
If you need support or clarification regarding the usage of Drogon in your project, visit the official Drogon support channel at [gitter](https://gitter.im/drogon-web/community)

Please create a new issue only if you think you have found a bug or if have a feature request/enhancement.

**Is your feature request related to a problem? Please describe.**
I am considering Drogon for a project but I am concerned that I have not found any reference to concurrency control in the ORM docs (pessimistic or optimistic).

**Describe the solution you'd like**
I would like concurrency control for the ORM. Currently, as far as I am aware, there is no concurrency control.
If there is one, does it use optimistic or pessimistic concurrency control?

**Describe alternatives you've considered**
The company Code Synthesis offers C++ ORM called ODB. They implement [optimistic concurrency control](https://www.codesynthesis.com/products/odb/doc/manual.xhtml#12). Unfortunately they do not offer an asynchronous interface for their ORM.