They're not in the duplicated handshake state machines anyway. But we still
shouldn't negotiate them. d1_pkt.c assumes Finished is the only post-CCS
handshake message. An unexpected handshake message in the current epoch may
either be a retransmit/out-of-order message from the previous handshake, or a
message from the next handshake (also potentially out-of-order). In the former
case, we shouldn't spin up another handshake state machine instance.

(This assumption is required due to a protocol bug. DTLS resets sequence
numbers after a handshake, so it is necessary to categorize handshake fragments
by pre-CCS and post-CCS to distinguish between retransmit and renego.)

Change-Id: Ib3c1c7085c729e36a40f7ff14494733156924a24
Reviewed-on: https://boringssl-review.googlesource.com/3028
Reviewed-by: Adam Langley <agl@google.com>