Create a new 'MultiExecProcNode' call API for plan nodes that don't

Create a new 'MultiExecProcNode' call API for plan nodes that don't
return just a single tuple at a time.  Currently the only such node
type is Hash, but I expect we will soon have indexscans that can return
tuple bitmaps.  A side benefit is that EXPLAIN ANALYZE now shows the
correct tuple count for a Hash node.