add parallel benchmark

[rogpeppe]

When many UUIDs are being generated concurrently,
contention on the atomic counter can slow things down.

There might be ways to speed this up, but for now, just
add a parallel benchmark so we can measure the baseline.

Initial results with go test -bench . -cpu 1,2 on my machine (two physical cores):

BenchmarkContended     	56413502	        18.3 ns/op
BenchmarkContended-2   	82533951	        33.5 ns/op

Note that the 33.5ns/op measure is worse than it appears, because
that's wall ns/op, not cpu-ns/op (see golang/go#31884), so the time is actually 67.0ns/op when contended.