Mirrors SSL_SESSION_to_bytes. It avoids having to deal with object-reuse, the
non-size_t length parameter, and trailing data. Both it and the object-reuse
variant back onto an unexposed SSL_SESSION_parse which reads a CBS.
Note that this changes the object reuse story slightly. It's now merely an
optional output pointer that frees its old contents. No d2i_SSL_SESSION
consumer in Google that's built does reuse, much less reuse with the assumption
that the top-level object won't be overridden.
Change-Id: I5cb8522f96909bb222cab0f342423f2dd7814282
Reviewed-on: https://boringssl-review.googlesource.com/5121
Reviewed-by: Adam Langley <agl@google.com>
fd67aa8c95