runner: Refactor handshake fragmenting slightly.

No behavior change. This is in preparation for buffering a flight of handshake
messages to reorder vigorously on flush.

Change-Id: Ic348829b340bf58d28f332027646559cb11046ac
Reviewed-on: https://boringssl-review.googlesource.com/3215
Reviewed-by: Adam Langley <agl@google.com>