Enable nested loop for gprestore connections

Statistics backups created in gpbackup versions starting from 1.30.5_arenadata16
have inefficient SQL for deleting existing statistics statements. For new
backups, this issue has been fixed in the previous commit. This patch fixes the
issue for existing backups by enabling nested loop join. This should lead to a
more efficient plan with an index scan instead of a seq scan.

Author: KnightMurloc

restore/wrappers.go

@@ -89,6 +89,7 @@ SET client_min_messages = error;
 SET standard_conforming_strings = on;
 SET default_with_oids = off;
 SET optimizer = off;
+SET enable_nestloop = on;
 `
 
 	setupQuery += "SET gp_ignore_error_table = on;\n"