Don't remove cached presences in `afterTearDown` event.
We might reconnect again and resume the session, in which case we need
that data still.

Instead, we remove cached presences when `clearSession` fires, because
then we know we're not reconnecting.

When reconnecting, we don't fetch presences from cache, we still have
them (since we no longer remove them in `afterTearDown`).

When reconnecting and not resuming, we clear the presences from cache,
since we're starting a new session.