update Ancestry writers

[kbrock]

extracted from #600

These are now non-editable class level variables:

• ancestry_base_class
• ancestry_column
• ancestry_delimiter (recently introduced #597 )

If this breaks in developer code try the class variable e.g.: self.class.ancestry_column

[kbrock]

update:

• found one more spot accessing instance variable ancestry_base_class
• fixed sql strategy accessors