Improved node traffic counters.

Improved node traffic counters.

Drop counters for packets, keep byte counters. But distinguish between
application data (channel traffic) and meta-data (including UDP probes).
Also track how much data we forward for other nodes.

The counters can be read by devtool_get_node_status(), and a new function
devtool_reset_node_counters() has been added that will zero the counters.