expr: add set_elem_expr as container for set element attributes

expr: add set_elem_expr as container for set element attributes

Add a new expression type "set_elem_expr" that is used as container for
the key in order to attach different attributes, such as timeout values,
to the key.

The expression hierarchy is as follows:

Sets:

    elem
     |
    key

Maps:

   mapping
   /      \
 elem    data
  |
 key

Signed-off-by: Patrick McHardy <kaber@trash.net>