UDP resolve enable in lo_server struct

While profiling my application (OSC via UDP), I've found that liblo spends a considerable amount (in my case its 1/3 of CPU time at least, and I'm doing a lot of computation) of time resolving the address of an incoming packet (getnameinfo in src/server.c).

Some users may just not care about where an UDP packet was coming from.
This patch introduces a switch that can turn this resolution step off for servers using UDP.