Fix buffer refcount leak with FDW bulk inserts

 Fix buffer refcount leak with FDW bulk inserts
 
 The leak would show up when using batch inserts with foreign tables
 included in a partition tree, as the slots used in the batch were not
 reset once processed.  In order to fix this problem, some
 ExecClearTuple() are added to clean up the slots used once a batch is
 filled and processed, mapping with the number of slots currently in use
 as tracked by the counter ri_NumSlots.
 
 This buffer refcount leak has been introduced in b676ac4 with the
 addition of the executor facility to improve bulk inserts for FDWs, so
 backpatch down to 14.
 
 Alexander has provided the patch (slightly modified by me).  The test
 for postgres_fdw comes from me, based on the test case that the author
 has sent in the report.
 
 Author: Alexander Pyhalov
 Discussion: https://postgr.es/m/b035780a740efd38dc30790c76927255@postgrespro.ru
 Backpatch-through: 14