Better manage subscriptionCounters (#14608)

Before this change:
- unsubscribe() was not called for a disconnection
- It seems that WebSocketClient calls connected() and reconnected().
  subscriptionCounters were incremented twice for a single reconnection,
  first from connected() and second from reconnected()

This might be a an additional change to
https://github.com/tootsuite/mastodon/pull/14579
to recover subscriptions after a reconnect.