cli: Add cli/debug split-key <rangeID>
cli: Add cli/debug split-key <rangeID>

For #5501. The result of running it against #5501 is a little boring:

```
./cockroach debug split-key ~/5501/data.ec2-xx-xxx-xxx-121.compute-1.amazonaws.com/ 54
[...]
better split: diff 14161 at /System/StatusNode/4/1458696823.168982825,0
better split: diff 6542 at /System/StatusNode/4/1458696813.172379557,0
better split: diff 1077 at /System/StatusNode/4/1458696803.185926328,0
No SplitKey found: storage/engine/mvcc.go:1821: the range cannot be split; considered range "/System/StatusNode/4"-"/System/\"tsd\\x12cr.node.exec.latency-10m-p90\\x00\\x01\\x89\\xf8\\x06.\\xc13\"" has no valid splits
```

So essentially we just happen to have a Range whose StartKey has so many
versions that it's chosen as the SplitKey, which isn't legal.

arjun4084346/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,vivekmenezes/cockroach,maxlang/cockroach,bg451/cockroach,maxlang/cockroach,cockroachdb/cockroach,glycerine/cockroach,paperstreet/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,paperstreet/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,glycerine/cockroach,veteranlu/cockroach,arjun4084346/cockroach,paperstreet/cockroach,cockroachdb/cockroach,glycerine/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,sunya123/cockroach,glycerine/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,petermattis/cockroach,veteranlu/cockroach,glycerine/cockroach,bg451/cockroach,maxlang/cockroach,maxlang/cockroach,maxlang/cockroach,bg451/cockroach,sunya123/cockroach,bg451/cockroach,petermattis/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,petermattis/cockroach,vivekmenezes/cockroach,glycerine/cockroach,veteranlu/cockroach,petermattis/cockroach,maxlang/cockroach,sunya123/cockroach,glycerine/cockroach,petermattis/cockroach,bg451/cockroach,arjun4084346/cockroach,veteranlu/cockroach,sunya123/cockroach,sunya123/cockroach,veteranlu/cockroach,cockroachdb/cockroach,sunya123/cockroach,vaibhavarora/cockroach,bg451/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,glycerine/cockroach,cockroachdb/cockroach,veteranlu/cockroach,bg451/cockroach,vaibhavarora/cockroach,maxlang/cockroach,arjun4084346/cockroach