Add reset() to Reservoir API

[ryantxu]

It would be nice to be able to reset the metrics counter.

I think the the most approprate way is to add a 'reset()' function on the Reservoir API.

If this seems like a reasonable direction, I will put together a pull request

Thanks
Ryan

[codahale]

This is a common request, and one I keep turning down. Resettable metrics mean a whole other layer of mutability which ends up producing a lot of weird edge cases.