re-write Endpoint.toString in terms of the cheaper getHostAddress

Using toString from InetSocketAddress will cause a DNS lookup to resolve the IP
to a hostname. An endpoint is defined as an IP, port, and service name, it's
unnecessary for it to resolve the hostname when printing its string
representation. A user should explicitly request the hostname via
getInetSocketAddress if they are interested.

Author: @sprsquish
Fixes #357
URL: https://github.com/twitter/zipkin/pull/357