Port early callback support to CBS.

Resolve one of the TODOs since it's quick. Adjust the
-expect-server-name test to assert it both in the normal codepath and
in the early callback, to provide test coverage for
SSL_early_callback_ctx_extension_get.

Change-Id: I4d71158b9fd2f4fbb54d3e51184bd25d117bdc91
Reviewed-on: https://boringssl-review.googlesource.com/1120
Reviewed-by: Adam Langley <agl@google.com>