Relations endpoint error when passed sync tokens

[clokep]

A next_batch token from /sync should be usable on the /relations endpoint. This was missing a test in complement, but now has one at matrix-org/complement#586. Dendrite errors on this test:

room_relations_test.go:266: CSAPI.MustDoFunc GET http://127.0.0.1:32983/_matrix/client/v1/rooms/%21ZeEvqcyz84VeEjw9:hs1/relations/$eQNMO057jy_F8f-ClA6m8Bi7UsTbNrnYX2-f4eID4BU?dir=f&limit=3&to=s11_0_0_0_0_1_1_0_1 returned non-2xx code: 500 Internal Server Error - body: {"message":"strconv.Atoi: parsing \"s11_0_0_0_0_1_1_0_1\": invalid syntax"}

Relevant spec:

from: The pagination token to start returning results from. If not supplied, results start at the most recent topological event known to the server.

Can be a next_batch or prev_batch token from a previous call, or a returned start token from /messages, or a next_batch token from /sync.