proof of concept. relationships.JoinCondition is given

proof of concept.   relationships.JoinCondition is given
everything known about the relationship in an ORM-agnostic
way.   The primaryjoin is determined as before, but is
then immediately parsed and annotated with "foreign",
"local" and "remote" annotations.  These annotations
will then drive the additional decisions made, namely
"direction" and "sync pairs".   The end user will be
able to pass a pre-annotated join condition in which
case those annotations take effect as given, which
will in one pass fix both #1401 as well as #610
(for #610, the "foreign" / "remote" annotation can
recursively descend down to ColumnElement objects).