Pipelined GroupLookup problems inside nested loops

It scans input bindings ahead and opens a group cursor for each (up to a limit). But it should only do this input open when the input bindings are at the same depth as the operator was itself opened. That is, it should remember but otherwise ignore outer loop bindings.

The other pipelining cursors are implemented based on LookaheadLeafCursor (GroupLookup isn't a leaf), which has a similar test for its openACursor inside open.

A new IT runs the existing GroupLookup Map IT inside yet another loop, failing before the change and passing after.