request #7604 - Iterative approach for computed values

Compute "per depth in the graph", with this approach their should be only
one SQL query per depth in the graph.

For history, goes from 17000 SQL requests to 1400

Change-Id: If6f344e5d636ef88fe3e4ddde725eaa9f59c8c41
Merge commit 'refs/changes/05/3205/4' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7604 - Iterative approach for computed values

Change-Id: I18ae610adec48f9172d187476894427ea0084988
This is Tuleap 7.6.99.55

Change-Id: I46aef002f6a165c4a5f4608bb28ff5946b8fd5f5