derp: remove a client round-trip waiting on serverInfo

It just has a version number in it and it's not really needed.
Instead just return it as a normal Recv message type for those
that care (currently only tests).

Updates #150 (in that it shares the same goal: initial DERP latency)
Updates #199 (in that it removes some DERP versioning)

derp/derp.go

@@ -209,13 +209,6 @@ func writeFrame(bw *bufio.Writer, t frameType, b []byte) error {
 	return bw.Flush()
 }
 
-func minInt(a, b int) int {
-	if a < b {
-		return a
-	}
-	return b
-}
-
 func minUint32(a, b uint32) uint32 {
 	if a < b {
 		return a