Currently, we have a bit of a convoluted flow where when sending
attestations, we start broadcasting them over gossip then pass them to
the attestation validation to include them in the local attestation pool
- it should be the other way around: we should be checking attestations
_before_ gossipping them - this serves as an additional safety net to
ensure that we don't publish junk - this becomes more important when
publishing attestations from the API.
Also, the REST API was performing its own validation meaning
attestations coming from REST would be validated twice - finally, the
JSON RPC wasn't pre-validating and would happily broadcast invalid
attestations.
* Unified attestation production pipeline with the same flow for gossip,
locally and API-produced attestations: all are now validated and entered
into the pool, then broadcast/republished
* Refactor subnet handling with specific SubnetId alias, streamlining
where subnets are computed, avoiding the need to pass around the number
of active validators
* Move some of the subnet handling code to eth2_network
* Use BitArray throughout for subnet handling
867d8f3223