add feature to enable/disable legacy namespacing

@mheffner, 

So after a couple hours of tracking down "where are my lost statistics," I was pointed to this 'feature.'

https://github.com/etsy/statsd/blob/master/docs/namespacing.md

Essentially, namespacing conventions differ between pre 0.50 and 0.50+.  This PR enables a toggle to choose which convention is preferable.

-S