Tidy DTLS cookie callback types.

const-correctness, unsigned long -> size_t.

Change-Id: Ic0c2685a48a0f98396c5753b6077c6c0c3b92326
Reviewed-on: https://boringssl-review.googlesource.com/1540
Reviewed-by: Adam Langley <agl@google.com>
6 files changed