SI-8924 don't hold reference to list in iterator

Current implementation of LinearSeqLike.iterator holds
reference to complete sequence. This prevent garbage collecting
of List elements while we keep reference to iterator somewhere.

This commit removes reference from Iterator implementation. This
allow garbage collection of List while we iterating over its
elements (when there is no other references to List in our program).
Merge pull request #4061 from maxcom/SI-8924-list-iterator-oom

SI-8924 don't hold reference to list in iterator