Aggregation discards sync committee messages/attestations on delays when subnet wasn't created

[rodrigo-o]

This issue is a follow-up of #1301, that issue is fixed right now, receiving messages before having the subnet created doesn't crash LibP2P anymore, we just discard them. The Subnet info data structure is create at the time of attesting or broadcasting a sync committee message, in the second third of the slot, some delays could make some messages being lost on aggregations, this could probe to be an issue in some not-so-common cases where we skip the second third altogether due to a delay and go directly to the aggregation step which in our case would be empty even when we actually received messages and attestations.

This is not a priority be we should be mindful of this in case we encounter the issue.