MB-12999: Share iterator's AVL tree key comparisons with snapshots

If iterator tries to seek on a snapshot handle, it was doing an avl
tree search over an incorrect aux handle causing crashes
Fix by using a single universal key comparison context across
both snapshot as well as wal comparisons of iterator
Test case added with code coverage
Change-Id: I4b52a3b925a12166761ca984ee0326cab6860b4a