Optimize session cache flushing

Sort SSL_SESSION structures by timeout in the linked list.
Iterate over the linked list for timeout, stopping when no more
session can be flushed.
Do SSL_SESSION_free() outside of SSL_CTX lock
Update timeout upon use

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8687)