keys: fix pretty-printing of range local keys

[petermattis]

Before:

/Local/Range/"\xb2"/RangeDescriptor

After:

/Local/Range/Table/42/RangeDescriptor

[cockroach-teamcity]

This change is 

[CLAassistant]

All committers have signed the CLA.

[spencerkimball]

Is Table confusing in this context? Why not /Local/Range/ID=42/RangeDescriptor? Maybe I'm missing something.

[petermattis]

The format is /Local/Range/[key]/RangeDescriptor where [key] is any other valid key. It usually will not be a simple table ID, but instead the boundary between 2 ranges of a table (i.e. /Table/42/1/-5000/"foo", or something like: /System/"" which is the end of meta2.