make pdu batch tokens zeroith-indexed

src/api/client/context.rs

src/api/client/message.rs

src/api/client/relations.rs

src/api/client/sync/mod.rs

src/api/client/sync/v3.rs

src/api/server/backfill.rs

src/core/pdu/count.rs

src/service/rooms/timeline/data.rs

src/service/rooms/timeline/mod.rs