Fix implementation of priority update with list

Previous implementation relied on Mysql specific behviour (esp. order of insert
ids). The current one get rid of it.

/!\ Warning: if you read this in a `git bisect` session: starting this /!\
/!\          commit rank will never be continuous again                /!\

Change-Id: Ied0e34088ef28a0aa98d8d1c2d96e27ce1fd9882
Merge commit 'refs/changes/41/3341/3' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix implementation of priority update with list

Change-Id: If72164181dc33c67c01c9f78c2dadf41f54c32b5
This is Tuleap 7.7.99.70

Change-Id: I31e7ced0fc027e3bf2f9c04aab4e964dd62b652f