Update sync committee duty

[mehdi-aouadi]

Modified sync committee duty

Modified beacon block root

def get_sync_committee_message(state: BeaconState,
                               block_root: Root,
                               validator_index: ValidatorIndex,
                               privkey: int,
                               store: Store) -> SyncCommitteeMessage:
    epoch = get_current_epoch(state)
    domain = get_domain(state, DOMAIN_SYNC_COMMITTEE, epoch)
    signing_root = compute_signing_root(block_root, domain)
    signature = bls.Sign(privkey, signing_root)

    return SyncCommitteeMessage(
        slot=state.slot,
        beacon_block_root=get_attester_head(store, block_root),
        validator_index=validator_index,
        signature=signature,
    )