Bearer connection stats

[Aleksander Morgado]

The following patch series implements a new 'Stats' property in the Bearer object which reports connection statistics. The values are directly retrieved from the device (e.g. queried), as opposed to measuring the stats in the network interface.

This implementation is currently limited to:
  * Bytes TX/RX. Updating the implementation to support other types (e.g. packets TX/RX) is trivial.
  * Updates every 30s. Still not sure how to best allow modifying this value, maybe as an additional optional parameter when creating the Bearer properties? It's likely not critical right now, though.
  * QMI modems only for now, although this could be extended for any modem that reports runtime statistics, like e.g. Huawei devices and their ^DSFLOWRPT unsolicited messages.

Comments welcome.