TLS: Add functions for managing cached session state

TLS: Add functions for managing cached session state

The new tls_connection_set_success_data(),
tls_connection_set_success_data_resumed(),
tls_connection_get_success_data(), and tls_connection_remove_session()
functions can be used to mark cached sessions valid and to remove
invalid cached sessions. This commit is only adding empty functions. The
actual functionality will be implemented in followup commits.

Signed-off-by: Jouni Malinen <j@w1.fi>