Use fully-qualified `PersistedEventPosition` when returning `RoomsForUser` #1515

Job	Run time
Calculate list of debian distros	4s
Build wheels on ubuntu-20.04 for x86_64	1m 40s
Build sdist	0s
Build .deb packages (debian:sid)	4m 43s
Attach assets to release	1s
	6m 28s

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