Use 'like' instead of 'ilike' when searching on Reference fields
In http://hg.tryton.org/trytond/file/tip/trytond/model/fields/reference.py#l179 we use 'ilike' instead of 'like'.
A test in a real database with a tryton-generated query we can see that a query taking 6 seconds with ilike, drops to 3 seconds with like.
m2m and o2m fields, which also care about reference fields, already use 'like'.