One singular self._delete_task: remove _num_connections and deprecate prune_instances

[evnchn]

Motivation

TL-DR: If you want something to always get cleaned up, clean it up in one place, never switch PICs. Keep logic clean and firm by always cleaning up when the function elapses - only way to stop it is to cancel the function.

So a client should, ideally, self-destruct.

But now a client is destroyed in a variety of places, and only sometimes it actually destroys.

• Before socket connect: by Client.prune_instances
• After socket connect: by the _delete_tasks, which can be multiple, but we act on the last one because we only actually delete when self._num_connections[document_id] == 0

This is quite some technical debt (no we are not 🤡 enough to write this on day one). Some left from 2.x when we had auto-index client. This PR seeks to eliminate it.

Implementation

Removals and deprecations:

• No more _num_connections.
• No more _delete_tasks (plural).
• Deprecate Client.prune_instances (does nothing now)

Additions

• We do a singular _delete_task. Considering the above, only the last one matter anyways, so why bother.
• self._reset_self_delete: For share the logic between the existing socket-disconnect self-delete, and the before-connect 60-second self-delete

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
  • Pray to pytests
• Pytests are not necessary.
• Documentation is not necessary.
  • Possibly to warn the user if they manually call Client.prune_instances though.

[evnchn]

Wait but then would 8414e1d indicate that our code can possibly break existing user code which spawns Client before core.loop is ready, causing error like:

...
  File "/home/runner/work/nicegui/nicegui/nicegui/client.py", line 104, in __init__
    self._reset_self_delete(timeout=60.0)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/runner/work/nicegui/nicegui/nicegui/client.py", line 313, in _reset_self_delete
    self._delete_task = background_tasks.create(delete_content(), name=f'delete content {self.id}')
                        ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/nicegui/nicegui/nicegui/background_tasks.py", line 27, in create
    assert core.loop is not None
           ^^^^^^^^^^^^^^^^^^^^^
AssertionError

🤔

[evnchn]

The force-pushes are due to incomplete pylint fixes.

We can agree that force-push are for "commits you don't want people to see"