Support OneToOne BIDIRECTIONAL with JoinTable

[andyjefferson]

Refer to datanucleus/datanucleus-core#196 and https://en.wikibooks.org/wiki/Java_Persistence/OneToOne#Mapping_a_OneToOne_Using_a_Join_Table if wanting to provide such a thing. This is not part of the JPA spec, and would follow what we already do for a N-1 UNI relation with join table (where it does make more sense to allow it).

[andyjefferson]

This likely works with some 1-1 uni relations, since we map them to N-1 uni internally (and that allows join table). Needs testcase(s) defining with the precise situation for this to be looked at further

[andyjefferson]

1-1 uni join table works fine.

This issue should be for 1-1 BI JOIN TABLE relation.