(#2415) - add LEVEL_PREFIX option for riakdown etc.

Forgot to add this for riakdown etc.  Gonna open this up for testing, though, so we can validate that it actually works for *DOWN authors.
All right, I at least did a rename here, `trueDbName` -> `baseName`, since it's really just the name without the adapter prefix. Unfortunate that we now have two possible prefixes - the adapter prefix and the user-specified prefix.