Sync committee rewards as currently implemented significantly increase variance in proposer rewards: https://github.com/ethereum/eth2.0-specs/issues/2448
For example, if there are 200000 validators (6.4m ETH staked), then during each 1/4-eek (~54 hour) period there is a chance of 512/200000 that a validator will get accepted into the sync committee, so on average that will happen once every 200000/512 * 1/4 = 97.6 eeks, or close to two years. The payout of this "lottery" is 1/8 of that, or ~12.2 eeks (a bit less than four months) of revenue. This is much more severe than block proposing (a chance of 1/200000 per slot, or a lottery worth ~0.38 eeks of revenue once every ~3.05 eeks).
This PR makes three changes to cut make the sync committee lottery less drastic and bring variance closer in line with what is available from block proposing:
* Reduce the `SYNC_REWARD_WEIGHT` from 8 to 2
* Add a penalty for not participating in the sync committee, so that despite the first change the total net reward for participating vs not participating is only cut down by 2x
* Reduce the sync committee period from 1/4 eek to 1/8 eek (~27 hours)
With these three factors combined, the lottery reduces to ~1.5 eeks of revenue, on average occurring every ~48 eeks. Validators who are maximally unlucky (ie. never become part of a sync committee) only lose ~3.12% of their rewards instead of ~12.5%.
The compromises that this approach makes are:
* In the extreme case where >50% of proposers are operating efficiently, being in a sync committee becomes a net burden. However, this should be extremely rare, and in such cases validators would likely be suffering inactivity leak penalties anyway.
* Incentive to participate in a sync committee decreased by 2x (but this is IMO an improvement; sync committees are _not_ as important as proposals and deserve to have lower rewards)
* Minimum data syncing needed to maintain a light client increases by 2x (from 24 kB per 54 hours to 24 kB per 27 hours). A burden for on-chain light clients, but still insignificant for others.
