request #7067: fix performance issues while admin selectbox with lot of values

The issue comes from the fact that for each value we display in the
administration of the element, we need to know if user can hide it or if
she can delete it. Those two cases depends on semantic, workflow, usage,
etc.

The previous strategy was to execute two sql requests (containing lot of
union statements) for each value. This is bad for performance because it
doesn't scale well (it is linear): on my setup with a field containing 40
values, it took 6sec (~150ms * 40) to display the page.

The new approach is the following: whenever we request the db to know if
we can hide/delete a value I assume we will do the same request for the
sibling value of the field. Therefore, instead of asking for one value,
we ask for all values of the field, leveraging on MySQL index
performances.

The result is that it takes only 600ms to display the page with this
changeset.

Change-Id: Ic2358c0a2e27af06233a3bc9f952b744a290b275
Merge commit 'refs/changes/41/2641/3' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7067: fix performance issues while admin selectbox with lot of values

Change-Id: I57e3a5e347ef97e14a2807f9612f817d7c7fed48
This is Tuleap 7.1.99.49

Change-Id: I45768cc885569510fe598ae18035ca825ee2b58e