Loss of history following unexpected/unavoidable err_needreggednick

Spun off from:

* (with screenshots)
* "… Through no fault of mine: all history disappeared, and did not reappear after I reidentified."
* "… report a bug in the client. For element-web, attaching the debug logs next time it happens will likely help the developers.".

Given the loss, not my fault, this feels like a bug (`T-Defect`)

Detail from :

> … the history requests @grahamperrin's client sent after he rejoined:

```text
2021-04-25 19:58:43,304 - synapse.access.http.8146 - 393 - INFO - GET-3532826 - 88.105.96.80 - 8146 - {@grahamperrin:matrix.org} Processed request: 0.141sec/-0.000sec (0.050sec, 0.001sec) (0.005sec/0.087sec/9) 218B 200 "GET /_matrix/client/r0/rooms/!qZXgYtEIHagWHrEQQb%3Amatrix.org/messages?from=t1259814-1971203433_757284961_13387972_840135145_729647167_2261531_252083715_957747346_191565&limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D HTTP/1.1" "Mozilla/5.0 (X11; FreeBSD amd64; rv:88.0) Gecko/20100101 Firefox/88.0" [21 dbevts]
2021-04-25 19:58:43,502 - synapse.access.http.8148 - 393 - INFO - GET-3534413 - 88.105.96.80 - 8148 - {@grahamperrin:matrix.org} Processed request: 0.111sec/-0.000sec (0.037sec, 0.010sec) (0.002sec/0.082sec/8) 218B 200 "GET /_matrix/client/r0/rooms/!qZXgYtEIHagWHrEQQb%3Amatrix.org/messages?from=t1259794-1971188387_757284961_13387972_840135145_729647167_2261531_252083715_957747346_191565&limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D HTTP/1.1" "Mozilla/5.0 (X11; FreeBSD amd64; rv:88.0) Gecko/20100101 Firefox/88.0" [1 dbevts]
2021-04-25 19:58:43,902 - synapse.access.http.8149 - 393 - INFO - GET-3534808 - 88.105.96.80 - 8149 - {@grahamperrin:matrix.org} Processed request: 0.143sec/-0.000sec (0.055sec, 0.008sec) (0.004sec/0.083sec/10) 218B 200 "GET /_matrix/client/r0/rooms/!qZXgYtEIHagWHrEQQb%3Amatrix.org/messages?from=t1259774-1971181568_757284961_13387972_840135145_729647167_2261531_252083715_957747346_191565&limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D HTTP/1.1" "Mozilla/5.0 (X11; FreeBSD amd64; rv:88.0) Gecko/20100101 Firefox/88.0" [21 dbevts]
2021-04-25 19:58:44,163 - synapse.access.http.8111 - 393 - INFO - GET-3520103 - 88.105.96.80 - 8111 - {@grahamperrin:matrix.org} Processed request: 0.169sec/-0.000sec (0.056sec, 0.011sec) (0.010sec/0.101sec/10) 218B 200 "GET /_matrix/client/r0/rooms/!qZXgYtEIHagWHrEQQb%3Amatrix.org/messages?from=t1259754-1971173976_757284961_13387972_840135145_729647167_2261531_252083715_957747346_191565&limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D HTTP/1.1" "Mozilla/5.0 (X11; FreeBSD amd64; rv:88.0) Gecko/20100101 Firefox/88.0" [21 dbevts]
2021-04-25 19:58:46,584 - synapse.access.http.8090 - 393 - INFO - GET-3524648 - 88.105.96.80 - 8090 - {@grahamperrin:matrix.org} Processed request: 2.345sec/-0.000sec (0.052sec, 0.004sec) (0.032sec/0.188sec/8) 218B 200 "GET /_matrix/client/r0/rooms/!qZXgYtEIHagWHrEQQb%3Amatrix.org/messages?from=t1259734-1971170676_757284961_13387972_840135145_729647167_2261531_252083715_957747346_191565&limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D HTTP/1.1" "Mozilla/5.0 (X11; FreeBSD amd64; rv:88.0) Gecko/20100101 Firefox/88.0" [21 dbevts]
```

> It looks like what's going on here is that Synapse is working its way through the 1053 events in the room between @grahamperrin's kick on 24th April and his rejoin on 25th April that he is not allowed to see (so returning an empty resultset). What is not clear to me is why element decides to stop paginating - it should continue to send requests until it hits visible events, or you switch to a different room.