Support new alteration "exists" (equi-join)

[Seddryck]

It should be possible to filter a result-set based on the content of another result-set. At the moment only equi-join conditions should be supported but could be extended in the future.

All rows of the initial result-set having no corresponding row in the second result-set are filtering out.

<result-set>
  <query>
    select 'a' as f0, 'FOO' as f1, null as f2 union all select 'B', 'bar', 'quark'
  </query>
  <alteration>
    <exist>
      <result-set>
         <query>
             select 'FOO' as Ref, 'Boo' as f0 union all select 'quark', 'Boo'
         </query>
      </result-set>
      <condition>
           <mapping candidate="f1" reference="Ref" />
      </condition>
    </exist>
  </alteration>
</result-set>