SSL: automatic rotation of session ticket keys.

SSL: automatic rotation of session ticket keys.

As long as ssl_session_cache in shared memory is configured, session ticket
keys are now automatically generated in shared memory, and rotated
periodically.  This can be beneficial from forward secrecy point of view,
and also avoids increased CPU usage after configuration reloads.

This also helps BoringSSL to properly resume sessions in configurations
with multiple worker processes and no ssl_session_ticket_key directives,
as BoringSSL tries to automatically rotate session ticket keys and does
this independently in different worker processes, thus breaking session
resumption between worker processes.