Make EXPLAIN report maximum hashtable usage across multiple rescans.

 Make EXPLAIN report maximum hashtable usage across multiple rescans.
 
 Before discarding the old hash table in ExecReScanHashJoin, capture
 its statistics, ensuring that we report the maximum hashtable size
 across repeated rescans of the hash input relation.  We can repurpose
 the existing code for reporting hashtable size in parallel workers
 to help with this, making the patch pretty small.  This also ensures
 that if rescans happen within parallel workers, we get the correct
 maximums across all instances.
 
 Konstantin Knizhnik and Tom Lane, per diagnosis by Thomas Munro
 of a trouble report from Alvaro Herrera.
 
 Discussion: https://postgr.es/m/20200323165059.GA24950@alvherre.pgsql