Control Slot in summary cache

[Neylix]

Describe the problem you discovered

Actually if a beacon summary node receive a valid slot, it adds it into the summary cache ets table. But if a malicious node send multiple times a slot, the beacon summary node will add it again an again in the summary cache leading to a memory overflow and a crash.

Describe the solution you'd like

To avoid this error, when a beacon summary node receive a slot from a slot node, it can verify in the summary cache if there is already a slot of the same time from the same node. If so the node can aggregate the replication attestations from the new slot into the already existing slot.

[samuelmanzanera]

The addition of the node information in the SummaryCache is done in PR: Set network coordinates from BeaconChain. We can base the work on this change.