Better latency reporting towards Riemann

Use "basho_bench op ~s" as a prefix when reporting latency events
towards riemann. Makes it easier to filter on ops, and looks nicer too.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Simplify the Riemann stats writer

The Riemann stats writer takes no options, simplify the code a little.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>