True Non-Blocking Support

[darkfrog26]

Currently our asynchronous support is simply wrapping a database call with Future(...) and a little bit more niceness, but it isn't truly non-blocking, a thread is still running because JDBC is built around blocking calls. However, we could introduce an extension to ScalaRelational that support true non-blocking database requests with a project like this: https://github.com/mauricio/postgresql-async

That project only supports PostgreSQL and MySQL, but we might be able to introduce support for others in the future.

[darkfrog26]

The biggest hurdle is going to be keeping the Session alive across multiple asynchronously running database requests.