IRIS workaround & SQL compatability

[nickmitchko]

This fix provides a workaround of DEFAULT values in alter statements and rename statements along with other SQL compatibility. (Changes mostly in schema.py). There is also a framework to inject our own SQL compiler in operations.py but I left that out of this commit as the other issues are fixed by the schema.py.

• sql_rename_table = "ALTER TABLE %(old_table)s RENAME %(new_table)s"
  • Provides proper syntax to DJANGO for table rename statements
• sql_rename_column = ( "ALTER TABLE %(table)s ALTER COLUMN %(old_column)s RENAME %(new_column)s" )
  • Provides proper syntax to DJANGO for column rename statements
• DEFAULTs are supported on alter statements, two bugs prevents their casual use
  • ALTER TABLE <table> ADD COLUMN <col> <type> DEFAULT <value> doesn't work if <value> is a parameter and needs to be a prepared statement.
  • ALTER TABLE <table> ADD COLUMN <col> <type> DEFAULT <value> doesn't work if <col> is an escaped reserved keyword such as "interval"

    def skip_default_on_alter(self, field):
        """
        Some backends don't accept default values for certain columns types
        (i.e. MySQL longtext and longblob) in the ALTER COLUMN statement.
        """
        return True

    def skip_default(self, field):
        # return super().skip_default(field)
        return True```