Modify sync committee logic and parameters to reduce variance

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.

specs/altair/beacon-chain.md

@@ -86,10 +86,10 @@ Altair is the first beacon chain hard fork. Its main features are:
 
 | Name | Value |
 | - | - |
-| `TIMELY_SOURCE_WEIGHT` | `uint64(12)` |
-| `TIMELY_TARGET_WEIGHT` | `uint64(24)` |
-| `TIMELY_HEAD_WEIGHT` | `uint64(12)` |
-| `SYNC_REWARD_WEIGHT` | `uint64(8)` |
+| `TIMELY_SOURCE_WEIGHT` | `uint64(14)` |
+| `TIMELY_TARGET_WEIGHT` | `uint64(26)` |
+| `TIMELY_HEAD_WEIGHT` | `uint64(14)` |
+| `SYNC_REWARD_WEIGHT` | `uint64(2)` |
 | `PROPOSER_WEIGHT` | `uint64(8)` |
 | `WEIGHT_DENOMINATOR` | `uint64(64)` |
 
@@ -129,7 +129,7 @@ This patch updates a few configuration values to move penalty parameters closer
 | Name | Value | Unit | Duration |
 | - | - | - | - |
 | `SYNC_COMMITTEE_SIZE` | `uint64(2**9)` (= 512) | Validators | |
-| `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | `uint64(2**9)` (= 512) | epochs | ~54 hours |
+| `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | `uint64(2**8)` (= 256) | epochs | ~27 hours |
 
 ## Configuration
 
@@ -579,10 +579,12 @@ def process_sync_committee(state: BeaconState, aggregate: SyncAggregate) -> None
     # Apply participant and proposer rewards
     all_pubkeys = [v.pubkey for v in state.validators]
     committee_indices = [ValidatorIndex(all_pubkeys.index(pubkey)) for pubkey in state.current_sync_committee.pubkeys]
-    participant_indices = [index for index, bit in zip(committee_indices, aggregate.sync_committee_bits) if bit]
-    for participant_index in participant_indices:
+    for index, participation_bit in zip(committee_indices, aggregate.sync_committee_bits):
+        if participant_bit:
             increase_balance(state, participant_index, participant_reward)
             increase_balance(state, get_beacon_proposer_index(state), proposer_reward)
+        else:
+            decrease_balance(state, participant_index, participant_reward)
 ```
 
 ### Epoch processing