replication for `typing` stream restarted from 0 on restart

This issue has been migrated from [#8524](https://github.com/matrix-org/synapse/issues/8524).

---

After restarting a worker, it is emitting log lines as follows:

```
2020-10-12 18:11:14,942 - synapse.replication.tcp.handler - 520 - INFO - process-replication-data-1 - Fetching replication rows for 'typing' between 0 and 16463350
2020-10-12 18:11:14,943 - synapse.logging.opentracing - 307 - ERROR - process-replication-data-1 - There was no active span when trying to inject the span into a byte dict. Did you forget to start one or did a context slip?
2020-10-12 18:11:15,349 - synapse.http.client - 381 - INFO - process-replication-data-1 - Received response to GET http://10.103.0.2:8126/_synapse/replication/get_repl_stream_updates/typing/DdMdhASQZL?from_token=0&upto_token=16463350: 200
2020-10-12 18:11:15,384 - synapse.replication.tcp.handler - 520 - INFO - process-replication-data-1 - Fetching replication rows for 'typing' between 49300 and 16463350
2020-10-12 18:11:15,400 - synapse.logging.opentracing - 307 - ERROR - process-replication-data-1 - There was no active span when trying to inject the span into a byte dict. Did you forget to start one or did a context slip?
2020-10-12 18:11:15,748 - synapse.http.client - 381 - INFO - process-replication-data-1 - Received response to GET http://10.103.0.2:8126/_synapse/replication/get_repl_stream_updates/typing/McoEWmdCwi?from_token=49300&upto_token=16463350: 200
2020-10-12 18:11:15,784 - synapse.replication.tcp.handler - 520 - INFO - process-replication-data-1 - Fetching replication rows for 'typing' between 107693 and 16463350
2020-10-12 18:11:15,785 - synapse.logging.opentracing - 307 - ERROR - process-replication-data-1 - There was no active span when trying to inject the span into a byte dict. Did you forget to start one or did a context slip?
2020-10-12 18:11:16,629 - synapse.http.client - 381 - INFO - process-replication-data-1 - Received response to GET http://10.103.0.2:8126/_synapse/replication/get_repl_stream_updates/typing/PnoOikhuQZ?from_token=107693&upto_token=16463350: 200
2020-10-12 18:11:16,667 - synapse.replication.tcp.handler - 520 - INFO - process-replication-data-1 - Fetching replication rows for 'typing' between 160170 and 16463350
```

Apart from the context errors as per #8510, surely it shouldn't start from zero?