query sync duties for correct slot

VC currently misses sync committee duties for first slot of most epochs
because the 1 slot offset is not taken into account. Duties for the next
slot must be used during any given current slot. We use the correct slot
for processing the duty, but do not use the correct slot for fetching.

beacon_chain/validator_client/duties_service.nim

@@ -409,23 +409,24 @@ proc pollForSyncCommitteeDuties*(service: DutiesServiceRef) {.async.} =
   let vc = service.client
   let
     currentSlot = vc.getCurrentSlot().get(Slot(0))
-    currentEpoch = currentSlot.epoch()
+    # https://github.com/ethereum/consensus-specs/blob/v1.4.0-alpha.3/specs/altair/validator.md#sync-committee
+    dutySlot = currentSlot + 1
+    dutyEpoch = dutySlot.epoch()
 
   if vc.attachedValidators[].count() != 0:
     let
       dutyPeriods =
         block:
           var res: seq[tuple[epoch: Epoch, period: SyncCommitteePeriod]]
           let
-            currentPeriod = currentSlot.sync_committee_period()
-            lookaheadSlot = currentSlot +
-                            SUBSCRIPTION_LOOKAHEAD_EPOCHS * SLOTS_PER_EPOCH
-            lookaheadPeriod = lookaheadSlot.sync_committee_period()
+            dutyPeriod = dutySlot.sync_committee_period()
+            lookaheadEpoch = dutyEpoch + SUBSCRIPTION_LOOKAHEAD_EPOCHS
+            lookaheadPeriod = lookaheadEpoch.sync_committee_period()
           res.add(
-            (epoch: currentSlot.epoch(),
-             period: currentPeriod)
+            (epoch: dutyEpoch,
+             period: dutyPeriod)
           )
-          if lookaheadPeriod > currentPeriod:
+          if lookaheadPeriod > dutyPeriod:
             res.add(
               (epoch: lookaheadPeriod.start_epoch(),
                period: lookaheadPeriod)
@@ -446,7 +447,7 @@ proc pollForSyncCommitteeDuties*(service: DutiesServiceRef) {.async.} =
 
     if total == 0:
       debug "No new sync committee member's duties received",
-            slot = currentSlot
+            slot = dutySlot
 
     let subscriptions =
       block:
@@ -470,10 +471,10 @@ proc pollForSyncCommitteeDuties*(service: DutiesServiceRef) {.async.} =
       let res = await vc.prepareSyncCommitteeSubnets(subscriptions)
       if res == 0:
         warn "Failed to subscribe validators to sync committee subnets",
-             slot = currentSlot, epoch = currentEpoch,
+             slot = dutySlot, epoch = dutyEpoch,
              subscriptions_count = len(subscriptions)
 
-  service.pruneSyncCommitteeDuties(currentSlot)
+  service.pruneSyncCommitteeDuties(dutySlot)
 
 proc pruneBeaconProposers(service: DutiesServiceRef, epoch: Epoch) =
   let vc = service.client