Decide whether or not to request client certificates early.

This allows us to merge two of the ssl3_digest_cached_records calls which were
almost, but not completely, redundant. Also catches a missing case: the buffer
may be discarded if doing session resumption but otherwise enabling client
authentication.

BUG=492371

Change-Id: I78e9a4a9cca665e89899ef97b815454c6f5c7e02
Reviewed-on: https://boringssl-review.googlesource.com/4885
Reviewed-by: Adam Langley <agl@google.com>