references by uuid

[ElectricMaxxx]

This should wrap up #493

didn't implement the stuff, commented in code not to change.

But there are still the Questions of @dbu

I go top to button through the text/conversation, cause it is really hard to remember what the purpose was, even for me :-)

[ElectricMaxxx]

This will fail for sure now ... Did a lot of changes

• introduced a object hash - uuid map, which is filled when a document is registered by its uuid (on DocumentManager::getReference() call with proxies referencing documents by their uuid <- @dantleech purpose)
• introduced a uuid - path map to find documents by their path in identity map, which are managed by uuid

so all documents still live in identityMap by their id, but the id can be the absolute paths directly or uuids which needs to be converted into paths (uuid - id map).