Now we are passing the folder where the round instances are saved.
The problem is that calling utils::Check or utils::Assert on 1 or 2 nodes, shutdowns all of them. Only those should be shutdown and this will work. There maybe some other mechanism to shutdown a particular node. Tianqi?
updating mock. It now wraps the calls to sync and reads config from configuration file.
I believe it's better not to use the preprocessor directive, i.e. not to put any test code in the engine_tcp. I just call the mock in the test_allreduce file. It's a file purely for testing purposes, so it's fine to use the mock there.
