want 'concurrency' option to `forEachParallel`

[trentm]

See SAPI-266 for occasion where this is really wanted, but basically in general where the array of inputs can grow large, you always want a limitation on concurrency. The programmer could do this with vasync.queue right now, but it would reduce boilerplate for a common use case if supported on forEachParallel.

[trentm]

starter patch for discussion: https://gist.github.com/trentm/d90e61035c91fe7e489c