Fix bug introduced by microvacuum for GiST

 Fix bug introduced by microvacuum for GiST
 
 Commit 013ebc0a7b7ea9c1b1ab7a3d4dd75ea121ea8ba7 introduces microvacuum for
 GiST, deletetion of tuple marked LP_DEAD uses IndexPageMultiDelete while
 recovery code uses IndexPageTupleDelete in loop. This causes a difference
 in offset numbers of tuples to delete. Patch introduces usage of
 IndexPageMultiDelete in GiST except gistplacetopage() where only one tuple is
 deleted at once. That also slightly improve performance, because
 IndexPageMultiDelete is more effective.
 
 Patch changes WAL format, so bump wal page magic.
 
 Bug report from Jeff Janes
 Diagnostic and patch by Anastasia Lubennikova and me