Make min_parallel_relation_size's default value platform-independent.

The documentation states that the default value is 8MB, but this was
only true at BLCKSZ = 8kB, because the default was hard-coded as 1024.
Make the code match the docs by computing the default as 8MB/BLCKSZ.

Oversight in commit 75be6646, noted pursuant to a gripe from Peter E.

Discussion: <90634e20-097a-e4fd-67d5-fb2c42f0dd71@2ndquadrant.com>

src/backend/utils/misc/guc.c

@@ -2758,7 +2758,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS,
 		},
 		&min_parallel_relation_size,
-		1024, 0, INT_MAX / 3,
+		(8 * 1024 * 1024) / BLCKSZ, 0, INT_MAX / 3,
 		NULL, NULL, NULL
 	},