Performance tweaks to proxy

Performance Tweaks for Proxy
 - Handle Multi metric packets
   - only make new buffer from metric if its a multi-metric packet. 
   - fix issue where previously multi-metric packets weren't forwarded correctly
 - Don't create new socket for every packet we are going to send. 
   - This saves time and resources on the proxy box. 
   - If your statsd instances are on a different box its nicer on both servers ne tstacks by reducing src port changes.
 
TODO: dry up the multi metric vs. single metric code.