This bug led to wrong values of the use_count fields in some SEL_ARG
trees that triggered complains on the server side when executing the
test case for LP bug 800184 if a debug build of the server was used.
  
This was the result of the incomplete fix for bug 800184.
To complete it the following corrections had to be made:
- the copy constructor for SEL_TREE must call the new function incr_refs_all()
  instead of the function incr_refs(), because references to next key parts
  from any SEL_ARG tree belonging to the list of the first key part has to be
  adjusted.
- the method and_sel_tree of the class SEL_IMERGE must use the copy constructor
  of the SEL_TREE class to make a copy of its second argument before it ANDs it
  with any SEL_TREE tree from the processed SEL_IMERGE object.