crypto/tls: rotate session keys in older TLS versions

Also encode the certificates in a way that's more
consistent with TLS 1.3 (with a 24 byte length prefix).

Note that this will have an additional performance cost
requiring clients to do a full handshake every 7 days
where previously they were able to use the same ticket
indefinitely.

Updates #25256

Change-Id: Ic4d1ba0d92773c490b33b5f6c1320d557cc7347d
Reviewed-on: https://go-review.googlesource.com/c/go/+/231317
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

handshake_messages_test.go

@@ -309,6 +309,7 @@ func (*sessionState) Generate(rand *rand.Rand, size int) reflect.Value {
 	s.vers = uint16(rand.Intn(10000))
 	s.cipherSuite = uint16(rand.Intn(10000))
 	s.masterSecret = randomBytes(rand.Intn(100)+1, rand)
+	s.createdAt = uint64(rand.Int63())
 	for i := 0; i < rand.Intn(20); i++ {
 		s.certificates = append(s.certificates, randomBytes(rand.Intn(500)+1, rand))
 	}