add concatLimit [fixes #1426]

[hargasinski]

Adds the elusive concatLimit.

[hargasinski]

While I was playing around with the concat function, I noticed it doesn't preserve order. i.e.

async.concat([30, 15], function(val, cb) {
    setTimeout(function() {
        cb(null, [''+val]);
    }, val);
}, function(err, result) {
    console.log(result);
});

will log

[ '15', '30' ]

as opposed to

[ '30', '15' ]

Should this be changed to the match the behavior of functions like filter, groupBy or map, which preserve the order in result? Would that be considered a breaking change?

/cc @aearly @megawac

[aearly]

You're right -- it should be order-preserving. I think that would be a subtle breaking change, but perhaps not worth saving for 3.0. I think concat has very few users (judging by how long it took someone to notice concatLimit missing).

[hargasinski]

@aearly after thinking about it a little more, it could technically fall under a bug fix, i.e. a patch release. Either way, I'll open a PR for it either tonight or at some point tomorrow. Thanks!