When blocks and attestations arrive, they are SSZ-decoded twice: once
for validation and once for processing. This branch enqueues the decoded
block directly for processing, avoiding the second, slow
deserialization.
* move processing of blocks and attestations to queue
* ...and out from beacon_node
* split attestation processing into attestations and aggregates
* also updates metrics
* clean up logging to better follow the lifetime of gossip: arrival,
validation and processing
* drop attestations and aggregates if there are too many
* try to prioritise blocks and aggregates before single-validator
attestations
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