SortNode: add method that looks up the expressions referenced by the Sortnode, and returns them paired with ASC/DESC.
Expression:
 - add function isSimpleAccessReference() to check whether it is a coll.attr.attr
 - add function getAccessNRef() to retrieve pair("attr.attr", "coll"),
SortIndex optimizer:
 - use the new functionality to get better readable code.
rename indexes -> indices