SSL: optimized rotation of session ticket keys.

SSL: optimized rotation of session ticket keys.

Instead of syncing keys with shared memory on each ticket operation,
the code now does this only when the worker is going to change expiration
of the current key, or going to switch to a new key: that is, usually
at most once per second.

To do so without races, the code maintains 3 keys: current, previous,
and next.  If a worker will switch to the next key earlier, other workers
will still be able to decrypt new tickets, since they will be encrypted
with the next key.