Setup needs support benchmarks with multiple nodes.

[daviddias]

We will need to have benchmarks with more than one node, so that we can benchmark things like exchange and pubsub. These multiple node tests will not make sense for profiling, so a good strategy to isolate them might be to have ./suite/benchmark/ and ./suite/profile/ for specific benchmark and profiling tests respectively and running any test found at ./suite/ in both lands (to avoid duplication of code).

A way that this could be done is by changing the benchmark suite signature to look like:

module.exports = function (callback) {
  const nNodes = 1
  function benchmark (nodes, callback) {
    const ipfs = nodes[0]
    ipfs.files.add([{
      path: 'a.txt',
      content: new Buffer('a')
    }], callback)
  }
  callback(null, nNodes, benchmark)
}

This way the node spawning would still remain independent from running the benchmark.