api3: Pass int as callbacktype, rather than enum

int is more ABI-safe and shorter to type. We should convert most of our
APIs requesting 'enum' as a type over to 'int'.. that's another story
though

Change-Id: Id20755d14744a59b9fc8313d691b70121e738cae
Reviewed-on: http://review.couchbase.org/39038
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>