UDP cluster support for statsd addresses

From @jylin in https://github.com/lyft/envoy/pull/1019:

Would it make sense to try to make the "statsd_tcp_cluster_name" then support UDP too (i.e. a "statsd_udp_cluster_name")?

(in reference to avoiding hardcoding IP addresses in config files for statsd)

This seems reasonable from a config/API perspective, but will involve a bit of work to make work. Currently the cluster implementations such as LogicalDnsCluster are assuming TCP connections.

This is also related to #492 - if there was general support for UDP proxying we would be able to leverage that to do have UDP addressable clusters for statsd.