go.crypto/ssh: reuse packet buffer for channel writes.

Test that different extended data streams within a channel are
thread-safe.

benchmark             old MB/s     new MB/s     speedup
BenchmarkEndToEnd     79.26        87.98        1.11x

benchmark                          old allocs     new allocs     delta
BenchmarkEndToEnd                  110            73             -33.64%

benchmark                          old bytes     new bytes     delta
BenchmarkEndToEnd                  2605720       1299768       -50.12%

LGTM=dave, jpsugar
R=agl, dave, jpsugar
CC=golang-codereviews
https://golang.org/cl/136420043

ssh/mempipe_test.go

@@ -59,7 +59,9 @@ func (t *memTransport) writePacket(p []byte) error {
 	if t.write.eof {
 		return io.EOF
 	}
-	t.write.pending = append(t.write.pending, p)
+	c := make([]byte, len(p))
+	copy(c, p)
+	t.write.pending = append(t.write.pending, c)
 	t.write.Cond.Signal()
 	return nil
 }