Fix some more leaks

- Took recommendations from a static analyzer, cpychecker (gcc-python)
- Fixed leaked reference via PyIter_Next()
- As a result, refactored the sequence_next() function to always return
  a new reference (it INCREFs on list/tuple/dict)
Change-Id: Ic430be0993a6cfbe17026f14696c1bdc05c7ec7b
Reviewed-on: http://review.couchbase.org/26305
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>