Update nginx.ssl.conf

Unlikely to affect performance on long-stablished connections, where the ssl handshake end being negligible, but this can be improved a bit:

The ssl session timeout does not harm, allows clients to reconnect with the same ssl session negotiated last day, speeding up the costly ssl-handshake. Google and others use 1-2 days value for this.

On the other hand the session cache was way too big, 20M is enough for 80.000 ssl sessions, using 100m (400k sessions) when sessions only last for 10 minutes makes little sense.