Add support for multiple UDP + JSON input plugins

Support comes by way of [[input_plugins.udp_servers]] in config.
A single port will map to a single database.

* Update sample config to include [[input_plugins.udp_servers]] example
* Change configuration.Configuration to hold config for array of udp servers
* Change configuration.UdpInputPortString to accept a port int
* Modify udp.NewServer to accept listenAddress string, database string
* Modify server.Server struct to hold an array of udp.Server
* Modify server.NewServer to not create any udp.Server
* Update server.ListenAndServe to check and instantiate all instances of udp.Server