V2ClientHello is going to be ugly wherever we do it, but this hides it
behind the transport method table. It removes a place where the
handshake state machine reaches into ssl3_get_message's internal state.
ssl3_get_message will now silently translate V2ClientHellos into true
ClientHellos and manage the handshake hash appropriately.

Now the only accesses of init_buf from the handshake state machines are
to create and destroy the buffer.

Change-Id: I81467a038f6ac472a465eec7486a443fe50a98e1
Reviewed-on: https://boringssl-review.googlesource.com/8641
Reviewed-by: Adam Langley <agl@google.com>