Fix /initialSync error due to unhashable `RoomStreamToken` (#10827)

The deprecated /initialSync endpoint maintains a cache of responses,
using parameter values as part of the cache key. When a `from` or `to`
parameter is specified, it gets converted into a `StreamToken`, which
contains a `RoomStreamToken` and forms part of the cache key.
`RoomStreamToken`s need to be made hashable for this to work.