Experimental ID tracker implementation.
Add comments about the three major phases of osm2pgsql

Osm2pgsql progress is generally broken down into processing, pending ways/relations,
and postgresql activities, but from reading the code it's not obvious where these
start, nor that they're the most time-intensive parts of running the program.

This adds some comments to indicate where in the overall flow you are
Merge remote-tracking branch 'mapquest/kk_multiback' into pn_multiback
Add basic comments to run_internal_until
Merge remote-tracking branch 'mapquest/kk_multiback' into pn_multiback
Merge remote-tracking branch 'origin/pn_multiback' into kk_multiback