Race condition in doppelganger shutdown

[arnetheduck]

https://github.com/status-im/nimbus-eth2/blob/2932d3b808eafb84cd3ceac0a5cc9e7622263285/beacon_chain/validator_client/doppelganger_service.nim#L51 fires an event to signal that doppelganger shutdown should happen - if there are queued async attestation tasks at that moment for validators that have not been detected as doppelgangers, it may happen that they run even though they shouldn't (our policy is to shut down all validators on a single doppelganger trigger).