Flush method not part of interface IDogStatsd

[UrsMetz]

The PR #144 added Flush to DogStatsdService but it is not present in the interface IDogStatsd that that service implements. So code that uses the interface instead of the concrete implementation cannot flush without also disposing the service.

[ogaca-dd]

@UrsMetz : Thank you for reporting this issue. Indeed, Flush should be a member of IDogStatsd. I will add Flush to IDogStatsd in the next major release as based from Changes that affect compatibility, adding a member to an interface is a breaking change.