split up input Processing tests to compare same metrics vs new ones
split up input Processing tests to compare same metrics vs new ones

on my laptop:

MASTER:
BenchmarkProcess-8     2000000         718 ns/op
PASS
ok    github.com/raintank/metrictank/input  2.796s

HEAD:
go test -run='^$' -bench=.
BenchmarkProcessUniqueMetrics-8      1000000        2388 ns/op
BenchmarkProcessSameMetric-8         2000000         885 ns/op
PASS
ok    github.com/raintank/metrictank/input  6.081s

So we're a bit slower but carbon input should be a lot faster
(for which we don't have benchmarks) since it used to do regex matching
all the time

raintank/metrictank,raintank/raintank-metric,raintank/raintank-metric,raintank/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric