Mdns more fixes

[szupi-ipuzs]

I have noticed that if a service is not added to any net_if at the initial call to addServiceImpl(), then it is not stored at all. This is not correct, because a net_if might be nonexisting/down at the time, but later might be up and announced via mdns_netif_ext_status_callback. So all the services added via addServiceImpl should be cached always. While fixing this I have noticed that the many static vectors can really be merged into a vector of CachedService structure - it makes usage much easier and it cleans-up after itself.

Tested this with some extent using a unit test and on a real device.