fix: prevent 'afterUnloadDocument' being triggered immediately after onLoadDocument

[bencarletonn]

• Leading up to the call of the unloadDocument method of the Hocuspocus class, if the document no longer has any clients, the following events are guaranteed to occur in a synchronous nature: onStoreDocument hooks -> afterStoreDocument hooks -> unloadDocument
• Since the unloadDocument method includes an asynchronous call to trigger all afterUnloadDocument hooks without any await, or without extending the promise chain, the completion of the afterUnloadDocument hook is not guaranteed when the storeDocumentHooks returns a resolved promise.
• This, in turn, frees the event loop and allows the Hocuspocus server to handle different hooks whilst the logic in the afterUnloadDocument callback is incomplete.

A particular case can arise, prompted by an instant disconnect and reconnect of the websocket, where the afterUnloadDocument hook is triggered immediately after the onLoadDocument hook for a specific document, leading to possible unexpecting behavior.

In our example, we were using the 'afterUnloadDocument' to remove a document from a local cache. You can imagine the problems this has created when that document is removed from the cache despite being active.