When creating a field of type JSON, it will be automatically
converted to TEXT with CHECK (json_valid(`a`)), if there wasn't any
previous check for the column.

Additional things:
- Added two bug fixes that was found while testing JSON. These bug
  fixes has also been pushed to 10.3 (with a test case), but as they
  where minimal and needed to get this task done and tested, the fixes
  are repeated here.
  - CREATE TABLE ... SELECT drops constraints for columns that
    are both in the create and select part.
  - If one has both a default expression and check constraint for a
    column, one can get the error "Expression for field `a` is refering
    to uninitialized field `a`.
- Removed some duplicate MYSQL_PLUGIN_IMPORT symbols