Beef up ObserveInfo __repr__ and constants

- Provide LOGICALLY_DELETED
- Fix bugs in __repr__. Specifically, the current function was being
  used as the __str__ and not __repr__, and bugs would arise for some
constants.

Change-Id: Idb19f44e383721184ea57897d273339db5c04ce8
Reviewed-on: http://review.couchbase.org/27183
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>