[connman] refactored counters

Originally by Slava Monich <slava.monich@jolla.com>.

1. Replaced stats.c with stats2.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).