Support `availableSlotsProof` in status

[tahpot]

VIP 10 requires an availableSlotsProof to be generated in the /status endpoint.

This is used to provide an on-chain proof that a storage node has enough available slots to support being a fallback node for a node that is being removed from the network.

The message format is ${nodeId}/${availableSlots}/${timestamp}, where timestamp is unix epoch in seconds.