SVK::Mirror::Backend::SVNRaPipe - Transparent SVN::Ra requests pipelining

 my @req = (['rev_proplist', 3'], ['replay', 3 0, 1, 'EDITOR'])
 $generator = sub { shift @req };
 $pra = SVK::Mirror::Backend::SVNRaPipe->new($ra, $generator);

 $pra->rev_proplsit(3);
 $pra->replay(3, 0, 1, SVK::Editor->new);