pg_upgrade with parallel tablespace copying

Pg_upgrade by default (without --link) copies heap/index files from the
old to new cluster.  This patch implements parallel heap/index file
copying in pg_upgrade using the --jobs option.  It uses the same
infrastructure used for pg_upgrade parallel dump/restore.  Here are the
performance results:

               --- seconds ---
       GB        git    patched
        2       62.09    63.75
        4       95.93   107.22
        8      194.96   195.29
       16      494.38   348.93
       32      983.28   644.23
       64     2227.73  1244.08
      128     4735.83  2547.09

Because of the kernel cache, you only see a big win when the amount of
copy data exceeds the kernel cache.  For testing, I used a 24GB, 16-core
machine with two magnetic disks with one tablespace on each.  Using more
tablespaces would yield larger improvements.  My test script is
attached.

I consider this patch ready for application.  This is the last
pg_upgrade performance improvement idea I am considering.

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

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