Currently, collection cleanup code is only run when an added event
is received from the server. Where that fails is in scenarios where
a server-side collection turns empty while an affected users is
disconnected - and then reconnects. There's no removed (or updated)
event so no cleanup code is run and you have stale data.
This commit guarantees a stale data check is run whenever a subscription
is established again. The `added` check was also maintained, although
I'm not too sure anymore it's is still needed. That may need to be
revisited.
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