Export storage stats via the REST endpoint

[penberg]

We already have "rows read" in the REST endpoint, let's add storage stats there too.

[penberg]

One possible way to fix this is to add a storage_used() function on Stats that performs a query to the dbstat table but so that it does not increase the rows read metric.

[penberg]

Whoever implements this, please also add the storage metric to the server heartbeat payload.