spark.sql.tungsten.enabled will be the default value for both codegen and unsafe, they are kept internally for debug/testing.

cc marmbrus rxin

Author: Davies Liu <davies@databricks.com>

Closes #7998 from davies/tungsten and squashes the following commits:

c1c16da [Davies Liu] update doc
1a47be1 [Davies Liu] use tungsten.enabled for both of codegen/unsafe