Tree utils: prevent unwanted change in length of list of extra fields

The props list contains the names of user definable fields that are queried in
addition to some standard fields. The length of this list is used to map them to
the results returned by the database. However, before this list got overridden
in every iteration and since its length was changed to include the standard
fields, the length was wrong in the next iteration. This commit fixes this by
using a separate variable for the resulting mapping. This leaves the original
props list untouched.
Make user evaluation multi user review aware

To make the user evaluation admin tool support multi user reviews, this commit
makes it use always the newest review on a node to build epochs. Currently, all
reviews are taken into account, but this will be changed to make sure now "own"
reviews are included for the user being evaluated.

This change is part of the discussion in issue #580.