Warn for settings that don't work with viewonly=True

Warn for settings that don't work with viewonly=True

Setting persistence-related flags on :func:`.relationship` while also
setting viewonly=True will now emit a regular warning, as these flags do
not make sense for a viewonly=True relationship.   In particular, the
"cascade" settings have their own warning that is generated based on the
individual values, such as "delete, delete-orphan", that should not apply
to a viewonly relationship.   Note however that in the case of "cascade",
these settings are still erroneously taking effect even though the
relationship is set up as "viewonly".   In 1.4, all persistence-related
cascade settings will be disallowed on a viewonly=True relationship in
order to resolve this issue.

Fixes: #4993
Change-Id: I4b607a96a7de2ffa15303a27fd93c162a681556d