BookStateFor: StorageDescriptor<[Key: ParachainsInclusionAggregateMessageOrigin], {
    begin: number;
    count: number;
    end: number;
    message_count: bigint;
    ready_neighbours?: {
        next: ParachainsInclusionAggregateMessageOrigin;
        prev: ParachainsInclusionAggregateMessageOrigin;
    };
    size: bigint;
}, false>

The index of the first and last (non-empty) pages.