Fix buffer usage stats for nodes above Gather Merge.

 Fix buffer usage stats for nodes above Gather Merge.
 
 Commit 85c9d347 addressed a similar problem for Gather and Gather
 Merge nodes but forgot to account for nodes above parallel nodes.  This
 still works for nodes above Gather node because we shut down the workers
 for Gather node as soon as there are no more tuples.  We can do a similar
 thing for Gather Merge as well but it seems better to account for stats
 during nodes shutdown after completing the execution.
 
 Reported-by: Stéphane Lorek, Jehan-Guillaume de Rorthais
 Author: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
 Reviewed-by: Amit Kapila
 Backpatch-through: 10, where it was introduced
 Discussion: https://postgr.es/m/20200718160206.584532a2@firost