revisit cx_oracle unicode handling / CLOB doesn't honor convert_unicode

[Mike Bayer]

cx_oracle types that subclass _NativeUnicodeMixin but not _OracleUnicodeText are essentially text types where convert_unicode=True/'force' is entirely non functional. Even if the cx_oracle coerce_to_unicode flag is turned on, which we no longer recommend, a CLOB will never return unicode. this needs to be worked out so that the public flags at least do as expected.