This allows us to use CBB for all handshake messages. Now, SSL_PROTOCOL_METHOD
is responsible for implementing a trio of CBB-related hooks to assemble
handshake messages.

Change-Id: I144d3cac4f05b6637bf45d3f838673fc5c854405
Reviewed-on: https://boringssl-review.googlesource.com/8440
Reviewed-by: Adam Langley <agl@google.com>