inability to close WT_CURSORs might impact performance for new iterator implementation

[JackLiJXL]

currently once we get a new iterator for a graph, we do not have a handle for the WT_CURSOR used in graph. This means we can only close the specific WT_CURSOR from the iterator.

[JackLiJXL]

right now WT Cursors are not closed when iterators are destroyed, this makes us lose all handles to the cursor

[artorhem]

Having the cursor tied to the iterator class is by design. We don't want the iterator to access other cursors.

In the future, if a usecase arises where we do need this functionality, we will mark iterator classes as friends.