Support EVP_AEAD in DTLS.

This CL removes the last of the EVP_CIPHER codepath in ssl/. The dead code is
intentionally not pruned for ease of review, except in DTLS-only code where
adding new logic to support both, only to remove half, would be cumbersome.

Fixes made:
- dtls1_retransmit_state is taught to retain aead_write_ctx rather than
  enc_write_ctx.
- d1_pkt.c reserves space for the variable-length nonce when echoed into the
  packet.
- dtls1_do_write sizes the MTU based on EVP_AEAD max overhead.
- tls1_change_cipher_state_cipher should not free AEAD write contexts in DTLS.
  This matches the (rather confused) ownership for the EVP_CIPHER contexts.
  I've added a TODO to resolve this craziness.

A follow-up CL will remove all the resultant dead code.

Change-Id: I644557f4db53bbfb182950823ab96d5e4c908866
Reviewed-on: https://boringssl-review.googlesource.com/2699
Reviewed-by: Adam Langley <agl@google.com>
7 files changed