benchmark: make concurrent requests configurable

In http_bench.js, allow the concurrent requests per client
to be configurable.

This also changes the launch of clients to wait until all
forked servers are online. This eliminates spurious error
messages at the start of the run.

PR-URL: https://github.com/nodejs/io.js/pull/2068


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>