Tweak range_stats output format.
Revise Bud#id representation.

Rather than using a 2-element array, instead use a single integer: we assume
that we need to represent at most 2^16 IDs, so we left-shift the node's port
number 16 positions and OR it with the desired node-local ID. This happens to be
slightly more space efficient; more importantly, such IDs are recognized as
being dense by the current range compression code, without requiring extra
magic.