londiste.repair: add option to dump in parallel

[tarvip]

No description provided.

[yesnik]

@tarvip , hello! Could you please explain what does the option --parallel do? Why is it convenient to have this option at our disposal?

[tarvip]

This is used by repair. When tables are repaired then source and target tables are dumped to disk and then these dumps are compared later. Without --parallel option tables are dumped in serial. First source is dumped then target. Downside is that if you have large table then transaction on target can be opened for a long time as it is opened before starting source dump.
With --parallel option source and target dumps are created at the same time.

[yesnik]

Thank you, Tarvi ( @tarvip ), I got it. This option will be very helpful for us.
Marko ( @markokr ), hello! Is it possible to merge this pull request into master?