target sync committee aggregators to 16

[djrtwo]

@mcdee Did some quick math to show that the current configuration of TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE as 4 had an unacceptably high probability of sync committee subnets not having aggregators.

@hwwhww tossed the math and some options into this spreadsheet. Based on discussions, it looks like the choice of 16 is the right tradeoff to generally have a high chance of having at least one aggregator per slot, even on a daily basis.

Note, that 4 was naively selected to reduce chatter on yet another global channel, but assuming attestation aggregation is not already overwhelming, this increases global messages for ~6% (4 more 16 aggregator topics on top of the 64 attestation topics that also have 16 aggregator targets)

[djrtwo]

Thanks for the fix @hwwhww !

[hwwhww]

The logic LGTM.

And about if should TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE be in preset/config yaml file, TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE should be "constant" if SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT := 128 is also constant. We could:

1. Set that SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT should be a constant value. So we can add assert SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT == TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE * 4 to test_config_invariants.py;
2. Or, we can put TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE to preset file to remind the client devs of updating this field correspondingly in the future. (I'm not strongly for this time... but it's easy to forget...)

What do you think?

[djrtwo]

What does the restriction with (1) buy us?

[hwwhww]

What does the restriction with (1) buy us?

I was thinking that if we somehow update SYNC_COMMITTEE_SIZE or SYNC_COMMITTEE_SUBNET_COUNT for other reasons, we would be notified to increase TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE if needs. However, I just tried other numbers on the spreadsheet, the TARGET_AGGREGATORS_PER_SYNC_SUBCOMMITTEE = 16 should have guaranteed a high chance of aggregator selections with different committee sizes. So, it's not "required". 😅

[djrtwo]

going to merge as is for now!
thanks