display space-usage configs, space used, space available for your storage server

[warner]

It would be nice if the "Storage Server" sub-page of the Welcome Page would show an estimate of how much space the storage server is using. This estimate could be built by one of the share-crawlers, by adding up how much space is used in each prefix directory. A loose estimate could be maintained cheaply by summing the last-known-size for each prefixdir, and updating one prefixdir per time period (as all crawlers do). Since shares are placed pseudo-randomly, an even looser and cheaper estimate could be made by measuring a fraction of the prefixdirs and multiplying.

The bucketcrawler doesn't look inside the bucket directories, but the lease-crawler does. Let's add "space consumed" functionality to the lease-crawler, and display the results on the storage-server WUI page.

[davidsarah]

See also #671 (bring back sizelimit (i.e. max consumed, not min free)).

[zooko]

See also #1836, which is to get this information from leasedb instead of share crawler. Changing the title of this ticket to make it independent of where the information comes from.

[zooko]

See also #648 which is for the client to display to its user the space-usage/space-available information about storage servers that it knows about.

[zooko]

See also #666 which is for the server to display to its user the space used up by its clients. (Among several other features that are part of #666.)