Fixup vbucket APIs

- Make proper wrappers for some macros to retain ABI safety
- Mark functions with interface attributes

Change-Id: I83beffa2be75d83ac66e20a8db943f6cae29dfc3
Reviewed-on: http://review.couchbase.org/38524
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>