Reasons:

 - it adds an O(1) lookup index for accessing NMIPxConfig's addresses.
   Hence, operations like merge/intersect have now runtime O(n) instead
   of O(n^2).
   Arguably, we expect low numbers of addresses in general. For low
   numbers, the O(n^2) doesn't matter and quite likely in those cases
   the previous implementation was just fine -- maybe even faster.
   But the simple case works fine either way. It's important to scale
   well in the exceptional case.
 - the tracked objects can be shared between the various NMPI4Config,
   NMIP6Config instances with NMPlatform and everybody else.
 - the NMPObject can be treated generically, meaning it enables code to
   handle both IPv4 and IPv6, or addresses and routes. See for example
   _nm_ip_config_add_obj().
 - I want core to evolve to somewhere where we don't keep copies of
   NMPlatformIP4Address, et al. instances. Instead they shall all be
   shared. I hope this will reduce memory consumption (although tracking a
   reference consumes some memory too). Also, it shortcuts nmp_object_equal()
   when comparing the same object. Calling nmp_object_equal() on the
   identical objects would be a common case after the hash function
   pre-evaluates equality.