Change Dial to NewClient, implement buffered clients
Add benchmarking tests
Special-case inc from count for a 2x speedup

benchmark             old ns/op    new ns/op    delta
BenchmarkCount              484          472   -2.48%
BenchmarkInc                232          125  -46.12%
BenchmarkTime               337          330   -2.08%
BenchmarkGauge              440          427   -2.95%
BenchmarkSet                125          123   -1.60%
BenchmarkCountProb           88           84   -4.29%
BenchmarkIncProb             86           72  -15.56%