Add parallel pg_dump option.

New infrastructure is added which creates a set number of workers
(threads on Windows, forked processes on Unix). Jobs are then
handed out to these workers by the master process as needed.
pg_restore is adjusted to use this new infrastructure in place of the
old setup which created a new worker for each step on the fly. Parallel
dumps acquire a snapshot clone in order to stay consistent, if
available.

The parallel option is selected by the -j / --jobs command line
parameter of pg_dump.

Joachim Wieland, lightly editorialized by Andrew Dunstan.

doc/src/sgml/perform.sgml

@@ -1433,6 +1433,15 @@ SELECT * FROM x, y, a, b, c WHERE something AND somethingelse;
        base backup.
       </para>
      </listitem>
+     <listitem>
+      <para>
+       Experiment with the parallel dump and restore modes of both
+       <application>pg_dump</> and <application>pg_restore</> and find the
+       optimal number of concurrent jobs to use. Dumping and restoring in
+       parallel by means of the <option>-j</> option should give you a
+       significantly higher performance over the serial mode.
+      </para>
+     </listitem>
      <listitem>
       <para>
        Consider whether the whole dump should be restored as a single