Add ResettableRateCounter

[paulbellamy]

#23

There is a Counter.Reset() method, but not a RateCounter.Reset() method. Making the RateCounter atomically resettable adds some performance overhead, so I've added a new ResettableRateCounter, which adds this overhead but has a Reset() method.

@cleeland, does this work for you?

Benchmark on my machine for reference:

Benchmark	Normal	Resettable	Difference
Basic	330.5 ns/op	518.3 ns/op	+56.8%
Parallel	467.1 ns/op	468.3 ns/op	+0.2%
With5MillionExisting	305.5 ns/op	507.8 ns/op	+66.2%