OGM-609 Adding performance test suite

@DavideD, @emmanuelbernard: I'm not yet completely done, but can you check it out and let me know what you think. To run the suite, do the following:

* mvn generate-sources -pl performance (generates the JMH benchmark code)
* configure the MongoDB host in `native-settings.properties` and `persistence.xml`, respectively
* run `org.hibernate.ogm.perftest.Launcher`, e.g. from your IDE; optionally adapt the `include()` call to match a subset of benchmark classes/methods only

Could you run the benchmark and let me know whether you think it's sound, test cases should be added etc. Personally I think the resulting numbers are ok-ish, at least not catastrophically bad from what I can say. I'm planning to add a test case for assocations.