Improves CTLOG_STORE setters

Changes them to have clearer ownership semantics, as suggested in
https://github.com/openssl/openssl/pull/1372#discussion_r73232196

.

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1408)