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>