Add back integration tests with Vert.x

[DavideD]

We used to have an integration test to check how Hibernate Reactive can be used with Vert.x.

We should add it back. Aside from testing functionality, it's also helpful for quick benchmarks and to see what happens under load.

On the same note, we should add a test that emulates the TechEmpower benchmark. Not to use for performances, but to prevent and debug errors related to the test.