refactor: Introduce type hinting in Connection class

[DannyvdSluijs]

In an effort / discovery to see if this library could be improved with the PHP Type Hints. This is adding them to the Connection class where the type was very clear. There are still a lot of mixed and missing types but I don't want to introduce any breakage beyond what the docblocks already pointed out.

I'm curious about your opinions and remarks to see if we should move this forward and if so in which way (small increments or one big bang commit).

[stephangroen]

Thanks @DannyvdSluijs. I think this is very welcome. It doesn't have to be one big bang commit, unless someone want to put in the effort all at once.