Fix ServiceTrackerMap reference count

Hey @rotty3000, 

I was looking into this that we discussed yesterday and I was tempted to leave it untouched. This was so because, to our needs, it does not matter to have the reference counter artificially high, as long as it never reaches zero (the map will hold a reference to the service until it gets unreferenced, so no need to count references here). 

Nevertheless, looking into the equinox reference implementation we could see that it has a limit of MAX_INT and then it would throw an exception, so it is better to keep it tidy and call getService just once per reference.