Send beacon slot if there is at least 1 replication attestation

[Neylix]

Describe the problem you discovered

Actually a node will send its slots only if it is a beacon node for this slot time. But since this PR #923, some nodes can have remaining transaction in their slot even if they are not beacon node for this slot time. They must send these slots to the next beacon summary node to ensure this transaction will be added the the beacon chain.

Describe the solution you'd like

In the subset module there is a function beacon_slot_node? this function should not block the sending of the slot if it is not empty. But this function should be a control to add or not the p2p view in the slot. This means only beacon node will fill the p2p view.

So before sending the slot, the function Slot.empty? will return true if the slot does not have transaction and the node is not beacon slot node (as the p2p view will not be filled)