Set rwstate inside ssl3_get_client_hello.

This is more consistent with other asynchronous hooks and gets it working in
DTLS.

Change-Id: Ia17d9d23910e8665b2756516ba729dffc79af8c0
Reviewed-on: https://boringssl-review.googlesource.com/3346
Reviewed-by: Adam Langley <agl@google.com>