NestedLoopStage incorrectly disables slots used during openInner:

https://github.com/10gen/mongo/blob/master/src/mongo/db/exec/sbe/stages/loop_join.cpp#L214

This occurs when inner side of the loop has blocking stage that iterates over its children during open(), like:

NLJ(1) 
  left:
    scan A
  right:
    hashAgg
       2: NLJ(2)
         left:
           scan B
         right:
           scan C 

when NLJ(1) is doing openInner, and hashAgg is doing getNext() and scan C is yielding, then once scan C completes its getNext, its slots will be fine, but scan B will have invalid pointer as it didn't make the copy, and that pointer will be invalid until NLJ(2) calls getNextOuterSide.

Proposed changes:

1. remove _outerGetNext flag. _outerGetNext is true if:
   • inner child was never opened - we don't need to disable slots, as the state is empty.
   •   We are reading outer side. In this case the _isReadingLeftSide == true, making _outerGetNext redundant.
   • openInner() is in progress - we can't disable slots here as the inner side slots may be currently used
2. add regression unit tests to prevent similar issue in the future.