Harmonize MongoDB sync and reactive client factories

[snicoll]

Now that we use the new MongoDB driver, an harmonization between the imperative and reactive variants is possible.

@christophstrobl already started that in his PR but this has some impact on the regular variant that needs to be reviewed.

There is some note about v2 vs. v3 where v3 does not allow you to set host/port which I think isn't accurate. If we manage to harmonize, only one factory could be used to create both clients as a BiFucntion<MongoClientSettings, MongoDriverInformation> can be mapped to MongoClients::create.