Unsubscribe by attributes should only match a subset of subscriptions

[OrKoN]

In #828 I also overlooked that removal of subscriptions should only apply to the first matching subscriptions to be backward compatible. This PR adds steps to account for that and cover the cases like the following:

For events:

subscribe(events=[A, B])
unsubscribe(events=[A]) => works
unsubscribe(events=[A, B]) => expected to throw

For contexts:

subscribe(contexts=[A, B])
unsubscribe(contexts=[A]) => works
unsubscribe(contexts=[A, B]) => expected to throw

subscribe(events=[A])
unsubscribe(events=[A], contexts=[A]) => expected to throw

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