[Update] Add dns cache in udp mode to ease the pressure of dns server.

[fatelei]

When using udp mode, the client will resolve ip address from dns server per request. It brings great pressure for my dns server. So I change the code, when using udp mode, resolve the ip address firstly.

[msiebuhr]

Another important detail here - this does completely sidestep DNS TTL, which may be quite bad to do.

[msiebuhr]

Also: If load on your DNS is a big thing, what about a local caching daemon? That way there's no need to maintain a caching DNS client in this codebase.

Thinking about all the things we then need to get right (we basically need to allow all DNS-relevant settings to be passed into statsd-client and get their implementation correct), having users do it outside the statsd-client might be a better solution.