Refactoring WorkspacesManager#getAllWorkspacesIdentifiers to return Set<String> instead of raw Set

 Refactoring WorkspacesManager#getAllWorkspacesIdentifiers to return Set<String> instead of raw Set
 -fixed all iterations of the returned set to use enhanced for loop instead of raw Iterator
 -avoiding unnecessary WorkspaceManager lookups in workspace id iteration loops
 -removing unnecessary calls to TreeSet constructor, since getAllWorkspacesIdentifiers already returns TreeSet