Update over Mandatory

Proposed feature.
Bug: https://bugs.freenetproject.org/view.php?id=434∞

Update over Mandatory is an enhancement to the auto-updater that allows it to pick up revocations, and updates, from the node's peers directly, rather than having to do actual freenet requests. The advantage of this is that it can work across the boundary between mandatory builds; the node requesting behaviour can be changed, but update over mandatory will still be compatible. This means that users who either don't run a 24x7 node, or don't let it update for a while, won't be left behind by the network. It is known that on 0.5 many users were left behind on old incompatible versions of freenet (at least, there are 0.5 public gateways which haven't been updated and therefore have no peers, or only connect to very old peers).

This is based on BinaryBlobs - they are required to transfer the signed keys that make up the revocation certificate, or the update.