optimize variable stinfo:

* uidrels/typerels sets become uidrel/typerel, and raise error if the rql
  use multiple uid/type restriction for a variable (wasn't properly handled
  anyway, should use IN)

* blocsimplification / optrelations / attrvars set are created only when
  necessary

-> up to 5 less set per variable killed, should improve rql cache memory
   foot print and somewhat speed-up annotation