MT no longer supports single-node keys

[Dieterbe]

this is due to #687

echo "single 1500367369 1500367369" | nc localhost 2003

with 0.7.3-18-g8d7570a this works fine,
but with 0.7.3-21-g7e9bf99 it results in:

metrictank_1   | panic: runtime error: slice bounds out of range
metrictank_1   | 
metrictank_1   | goroutine 441 [running]:
metrictank_1   | github.com/raintank/metrictank/idx/memory.(*MemoryIdx).add(0xc4204d6960, 0xc42182f8c0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/idx/memory/memory.go:200 +0x13ff
metrictank_1   | github.com/raintank/metrictank/idx/memory.(*MemoryIdx).AddOrUpdate(0xc4204d6960, 0xc42182f710, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/idx/memory/memory.go:117 +0x2da
metrictank_1   | github.com/raintank/metrictank/idx/cassandra.(*CasIdx).AddOrUpdate(0xc4204d6960, 0xc42182f710, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/idx/cassandra/cassandra.go:241 +0x1ba
metrictank_1   | github.com/raintank/metrictank/input.DefaultHandler.Process(0xc42037047c, 0xc420370480, 0xc420236cc0, 0xc420370484, 0xc420370488, 0xf9cea0, 0xc4204d65a0, 0xfa69c0, 0xc4204d6960, 0xc4201239e0, ...)
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/input/input.go:71 +0x19d
metrictank_1   | github.com/raintank/metrictank/input.(*DefaultHandler).Process(0xc4203ac280, 0xc42182f710, 0x0)
metrictank_1   | 	<autogenerated>:1 +0x60
metrictank_1   | github.com/raintank/metrictank/input/carbon.(*Carbon).handle(0xc4204d6600, 0xfa6720, 0xc4204d8080)
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/input/carbon/carbon.go:196 +0x42d
metrictank_1   | created by github.com/raintank/metrictank/input/carbon.(*Carbon).accept
metrictank_1   | 	/home/dieter/go/src/github.com/raintank/metrictank/input/carbon/carbon.go:139 +0xa6