[System.Threading.Tasks.Dataflow] Replace implementation with CoreFx version
We were seeing some random test failures on Jenkins with our Dataflow implementation.
Replacing it with Microsoft's CoreFx version fixed those and makes us more compatible.
While we'd ideally not ship this assembly at all with Mono (it doesn't ship with .NET),
we shipped it in the past and as such people might rely on it so we can't remove it.
The CoreFx commit this version was taken is
905a1940bcda0afdca2f14ceb2b0161ebc4d1d02.