datarace: redirect "Data race @ ..." messages to model-checker bug list

datarace: redirect "Data race @ ..." messages to model-checker bug list

Data race information was previously printed in multiple ways: through
the datarace.cc printRace() method and through the model-checker
assert_bug() method. Start unifying that.

Other notes:
- Data race objects probably should be freed (even though they are
  snapshotted). I fix this.
- Races could be printed more than once previously, since the
  unrealizedraces vector was never cleared of realized races. Now, we
  clear this vector every time we process it (in a feasible execution),
  either stashing a bug message or dropping the race entirely.