Isolate transient schema helper
Fix #283
Polymorphic lookup from all registered dataSources

Polymorphic model lookup was previously limited to the same dataSource
as the modelFrom model, which turns out to be too restrictive. This was
uncovered by the use of a Transient model, not being able to lookup a
PersistedModel.
Re-use modelBuilder - correctly fixes lookup
getTransientSchema => getTransientDataSource
Remove legacy Schema references
Merge pull request #286 from fabien/fix/polymorphic-transient

getTransientDataSource helper + embedsMany/polymorphic test case fix