runner: Remove block pool

runner is a fork from a very old version of Go's crypto/tls. Back then,
crypto/tls implemented some pooled memory thing to reduce allocations.
The abstraction is pretty confusing.

Replace it with (I think) more idiomatic patterns using []byte and
bytes.Buffer. As part of this, I've moved things common to TLS and DTLS
into the encrypt method so encrypt is now responsible for generating the
explicit IV and adding TLS 1.3 padding.

Change-Id: I21527dd406d2691bc5d24378a832114c43b8b753
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/71367
Reviewed-by: Nick Harper <[email protected]>
Commit-Queue: David Benjamin <[email protected]>

ssl/test/runner/common.go

@@ -1997,8 +1997,9 @@ type ProtocolBugs struct {
 	// session ID in the ServerHello.
 	DTLS13EchoSessionID bool
 
-	// DTLSUsePlaintextRecord header, if true, has DTLS connections never
-	// use the DTLS 1.3 record header.
+	// DTLSUsePlaintextRecord header, if true, has DTLS 1.3 connections to use
+	// the DTLS 1.2 record header once the handshake completes. The bug is not
+	// activated during the handshake so that the handshake can complete first.
 	DTLSUsePlaintextRecordHeader bool
 
 	// DTLS13RecordHeaderSetCIDBit, if true, sets the Connection ID bit in
@@ -2248,13 +2249,6 @@ func (c *Credential) signatureAlgorithms() []signatureAlgorithm {
 	return supportedSignatureAlgorithms
 }
 
-// A TLS record.
-type record struct {
-	contentType  recordType
-	major, minor uint8
-	payload      []byte
-}
-
 type handshakeMessage interface {
 	marshal() []byte
 	unmarshal([]byte) bool