parallel: more efficient Run

We refactor Run to make it faster and more memory efficient.

Also added some benchmarks to verify this:

benchmark                old ns/op     new ns/op     delta
BenchmarkRunSingle       3163          1933          -38.89%
BenchmarkRun1000p100     714010        379910        -46.79%

benchmark                old allocs     new allocs     delta
BenchmarkRunSingle       10             3              -70.00%
BenchmarkRun1000p100     3007           3              -99.90%

benchmark                old bytes     new bytes     delta
BenchmarkRunSingle       412           240           -41.75%
BenchmarkRun1000p100     48394         240           -99.50%

There may be some advantage to using http://golang.org/pkg/sync/atomic/#CompareAndSwapInt64 instead of a mutex here, although I suspect the effect would be negligible in most situations. However a benchmark would be better than my wild guessing.