PYCBC-139, PYCBC-132; fixup lock/unlock errors

- Also allow for an OperationResult to be passed as an argument to
  unlock_multi (is this needed?)

Change-Id: I9802fa4ebc220fb6ce8e393d39f9b1444381e1d4
Reviewed-on: http://review.couchbase.org/26990
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>