Refactored counters

1. Replaced stats.c with jolla-stats.c which isn't doing anything we don't need. The new implementation is much smaller and simpler. It also eliminates the global hashtable which served the purpose of associating service with stats - now it's done using a pointer. Roaming and home statistics are separated from each other.
2. Fixed the problem with service being notified about the counter changes for a wrong interface. The cellular service may be associated with different interfaces over time. In practice it's ether rmnet0 or rmnet1, depending on MMS context activity and whether or not one of those interfaces was up when connman started (when it crashes, ofono interfaces stay up).