Stork needs to show LPS statistic

With the completion of #226 (closed) we now have a dashboard that shows subnets and pool utilization. We should extend it with leases per second statistic. Here's a mockup made by @godfryd how this could possibly look like.

The ability to show it in the last 5 or 15 mins and some larger timescale (24h maybe) would be very useful.

This will require some small design. Here's couple caveats that's worth considering:

• should we keep the historic (last 24h) data in stork or in kea?
• what if Kea is restarted?
• if we implement something new in Kea, how can this work with older Kea releases?
• Kea doesn't have explicit LPS statistic, but has the ability to store multiple observations of addresses-assigned with timestamp. This can be used to estimate LPS. But is it precise enough? If there's a lot of traffic, the observations are only split second apart. If there's no traffic at all, there are no observations.
• Kea has a mechanism for storing multiple observations. If it's useful, perhaps we could use it. If not, maybe we could get rid of it altogether?