Reduce default value of max_prepared_transactions from 50 to 5. This

Reduce default value of max_prepared_transactions from 50 to 5.  This
saves nearly 700kB in the default shared memory segment size, which seems
worthwhile, and it is a feature that many users won't use anyway.  Per
Heikki's argument, there is no point in a compromise value --- those who
are using 2PC at all will probably want it at least equal to max_connections.
But we can't set it to zero by default without breaking the prepared_xacts
regression test.