Reverse calculate the radius at startup by using the furthest content item in the ContentDB

[kdeme]

Fluffy currently sets the radius at max when the node starts up.
Because the radius only gets winded down when the storage-capacity hits, this can take a bit of time for a node to drop.
Additionally, depending on which data (which distance) gets added since start-up, this could still linger at to high radius values for a while.

This gives a wrong perspective on the nodes their capacity on the network, which is for example visible on glados interface.

We can reverse calculate the radius based on the furthest Element (that code is already there for pruning). But there probably should be some extra conditions based on how full the node is. (e.g. only do this when the node is > 90% capacity, this is what Trin does afaik).