A novel optimisation for attestation and sync committee message
validation: when batching, we look for signatures of the same message
and aggregate these before batch-validating: this results in up to 60%
fewer signature verifications on a busy server, leading to a significant
reduction in CPU usage.
* increase batch size slightly which helps finding more aggregates
* add metrics for batch verification efficiency
* use simple `blsVerify` when there is only one signature to verify in
the batch, avoiding the RNG
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