Reduce number of SQL request for backlog content

On Tuleap backlog content reduce from 5600 SQL requests to 340.
Still high but worth the fix.

It also reduce the memory consumption (no longer need to increase max memory
limit to 512MB...)

Change-Id: If28c02165048f84723f2f6ab89fda6c36bf6b7ee
Merge commit 'refs/changes/00/1300/3' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Reduce number of SQL request for backlog content

Change-Id: I454653316afb433e85038d6c6ae71e3f0160fd50
Tuleap 6.5.99.54

Change-Id: Id80aa4706fb40b87d79527c4f8ed2223ccfd27c3