add timing information to pg_upgrade

I've been looking into some options for reducing the amount of downtime
required for pg_upgrade, and $SUBJECT seemed like something that would be
worthwhile independent of that effort.  The attached work-in-progress patch
adds the elapsed time spent in each step, which looks like this:

  Performing Consistency Checks
  -----------------------------
  Checking cluster versions                                   ok (took 0 ms)
  Checking database user is the install user                  ok (took 3 ms)
  Checking database connection settings                       ok (took 4 ms)
  Checking for prepared transactions                          ok (took 2 ms)
  Checking for system-defined composite types in user tables  ok (took 82 ms)
  Checking for reg* data types in user tables                 ok (took 55 ms)
  ...

This information can be used to better understand where the time is going
and to validate future improvements.  I'm open to suggestions on formatting
the timing information, assuming folks are interested in this idea.

Thoughts?

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com