fix: non-scalar fields in embeddedsmany broken after document save

Fields with types such as DateTime in embeddedsmany can't be re-set with setDocumentData after a save. This patch fixes the issue by reusing the documents in the embeddedgroup and marking them all as "saved" an "not modified".

Some assertions in tests needed to be changed, as the embeddedgroup is now supposed to be initialized after a save.