Server-side OCSP stapling support.

This is a simpler implementation than OpenSSL's, lacking responder IDs
and request extensions support. This mirrors the client implementation
already present.

Change-Id: I54592b60e0a708bfb003d491c9250401403c9e69
Reviewed-on: https://boringssl-review.googlesource.com/5700
Reviewed-by: Adam Langley <agl@google.com>