derp: reduce DERP memory use; don't require callers to pass in memory to use

The magicsock derpReader was holding onto 65KB for each DERP
connection forever, just in case.

Make the derp{,http}.Client be in charge of memory instead. It can
reuse its bufio.Reader buffer space.

cmd/derper/mesh.go

@@ -118,8 +118,7 @@ func runMeshClient(s *derp.Server, host string, c *derphttp.Client, logf logger.
 			return
 		}
 		for {
-			var buf [64 << 10]byte
-			m, connGen, err := c.RecvDetail(buf[:])
+			m, connGen, err := c.RecvDetail()
 			if err != nil {
 				clear()
 				logf("Recv: %v", err)