[PERF] Speed improvement in Search operation

A search operation based on a single attribute, like (uid=user.123), should not pass through the optimizer at all, as we don't need to compute the number of entries.

What we have now is that the search is built as a conjonction (AND) operation as if we have injected this filter :
(& (objectclass =* ) (uid = user.123))

The cost of doing such a count is not negligeable (5% of the overall operation)