Optional
validateA function that blockBrokers should call prior to returning a block to ensure it can maintain control of the block request flow. e.g. TrustedGatewayBlockBroker will use this to ensure that the block is valid from one of the gateways before assuming its work is done. If the block is not valid, it should try another gateway and WILL consider the gateway that returned the invalid blocks completely unreliable.
An optional list of peers known to host at least the root block of the DAG that will be fetched.
If this list is omitted, or if the peers cannot supply the root or any child blocks, a
findProviders
routing query will be run to find peers that can supply the blocks.