foreign keys relation improvements

[devkral]

Changes:

• progress with multiple primary keys
• cleanup of related_fields
• allow multiple related_names with the same name to the same object
• slightly not documented yet Prefetch syntax change: you have to use the foreign key name when traversing a foreign key
• docs, changelog

[tarsil]

slightly not documented yet Prefetch syntax change: you have to use the foreign key name when traversing a foreign key

We need properly documented. We can't merge with missing information I'm afraid.

[devkral]

can you check the information? I hope I found all references

[devkral]

I added a note concerning foreign key traversal + that there was a change. Can you refer it in the changelog?

[devkral]

I deleted the special attention section because it is obsolete. It explains the old traversal approach with its weaknesses