Tidy up retargeting code and debug.log output.

- remove unused int64_t retargetSpacing
- modulate timespan in an isolated int64_t nModulatedTimespan
- keep nActualTimespan intact, to retain the original value
- compress debug.log output from 5 into 3 lines while retaining
  all information.
Merge pull request #588 from patricklodder/1.8-dev-retargeting

Tidy up retargeting code and it's debug.log output