Add SSL_SESSION_to_bytes to replace i2d_SSL_SESSION.

Deprecate the old two-pass version of the function. If the ticket is too long,
replace it with a placeholder value but keep the connection working.

Change-Id: Ib9fdea66389b171862143d79b5540ea90a9bd5fb
Reviewed-on: https://boringssl-review.googlesource.com/2011
Reviewed-by: Adam Langley <agl@google.com>