MFV r306423: 7402 Create tunable to ignore hole_birth feature

Until we can resolve the numerous hole_birth bugs that have cropped up
recently, and come up with a way going forwards to protect users from
corruption, we should disable the hole_birth feature. Using a tunable
allows those who are confident that their data is correct to continue to
take advantage of the feature.

Closes #188

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Author: Paul Dagnelie <pcd@delphix.com>