remove not found uuids from documents to fetch

Just to avoid problems when creating documents from nodes.

Solved issue: #545
if https://github.com/doctrine/phpcr-odm/pull/546/files#r16286535 is about this comment: uuids is all uuid from the ids array. so there is no risk here when counting only the uuids i think, the result will never be different - but the performance is better.