Add udp connection reuse

[Mzack9999]

Closes #116

Example code:

rdnsClient, err := retryabledns.NewWithOptions(retryabledns.Options{
		BaseResolvers:         []string{"192.168.5.1:53", "8.8.8.8:53"},
		MaxRetries:            1,
		ConnectionPoolThreads: 10, // To enable connection pool
	})
if err != nil {
	log.Fatal(err)
}
defer rdnsClient.Close()
_, err := rdnsClient.QueryMultiple("example.com", []uint16{dns.TypeA})
if err != nil {
	log.Fatal(err)
}

[tarunKoyalwar]

lgtm !