Make Join Prefixes optional

This was requested before but it got lost. We should allow the JoinNode field name prefixes to be optional. In which case the user is responsible for making sure field names are unique. Also the default behavior should be safe so not having prefixes should be an explicit action instead of the absence of an action.

Start by locating the JoinNode implementation and its existing tests. Trace how field name prefixes are configured and verify that prefixes remain enabled by default while an explicit option disables them; completion should include coverage for both behaviors and the uniqueness responsibility described in the issue.

Written by the indexing model from the issue text.