Use fully-qualified `PersistedEventPosition` when returning `RoomsForUser` #1514

Job	Run time
Build wheels on ubuntu-20.04 for x86_64	1m 41s
Calculate list of debian distros	7s
Build sdist	0s
Build .deb packages (debian:sid)	6m 43s
Attach assets to release	0s
	8m 31s

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