Move Channel ID initialization out of ssl3_new.

Configuration data inherited from the ctx happens in SSL_new. (This also gets
in the way of using ssl3_free/ssl3_new to implement SSL_clear.)

Change-Id: I2773af91abf4e1edc0c1a324bc1e94088d7c2274
Reviewed-on: https://boringssl-review.googlesource.com/2821
Reviewed-by: Adam Langley <agl@google.com>