Pg_upgrade speed for many tables

Magnus reported that a customer with a million tables was finding
pg_upgrade slow.  I had never considered many table to be a problem, but
decided to test it.  I created a database with 2k tables like this:

    CREATE TABLE test1990 (x SERIAL);

Running the git version of pg_upgrade on that took 203 seconds.  Using
synchronous_commit=off dropped the time to 78 seconds.  This was tested
on magnetic disks with a write-through cache.  (No change on an SSD with
a super-capacitor.)

I don't see anything unsafe about having pg_upgrade use
synchronous_commit=off.  I could set it just for the pg_dump reload, but
it seems safe to just use it always.  We don't write to the old cluster,
and if pg_upgrade fails, you have to re-initdb the new cluster anyway.

Patch attached.  I think it should be applied to 9.2 as well.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +