Sends read receipts on login

[dbkr]

Somehow Riot/iOS sends read receipts for (some?) rooms when you log into the app which clears unread markers.

Evidence: I logged in this morning and now a load of my rooms (like the ones in Low Priority which I know I haven't viewed in ages) are marked as read.

Synapse log contains:

2018-06-29 08:43:04,164 - synapse.rest.client.v1.login - 178 - INFO - POST-2459201 - Got login request with identifier: {'type': 'm.id.user', 'user': 'dave'}, medium: None, address: None, user: 'dave'
[...]
2018-06-29 08:43:58,965 - synapse.access.http.8080 - 129 - INFO - POST-2465133 - 80.250.101.234 - 8080 - {@dave:matrix.org} Processed request: 0.281sec (0.000sec, 0.000sec) (0.006sec/0.014sec/6) 2B 200 "POST /_matrix/client/r0/rooms/!UcYsUzyxTGDxLBEvLz:matrix.org/read_markers?access_token=<redacted> HTTP/1.1" "Riot/0.6.17 (iPhone; iOS 11.3; Scale/2.00)" [0 dbevts]

[manuroe]

This seems to be a regression introduced by matrix-org/matrix-ios-sdk#430.