Warn about communication attempts with disconnected clients

[falkoschindler]

This PR tries to solve issue #3028 where a memory leak in the application code only becomes apparent when trying to delete a UI element. To warn the user about a deleted client that is still in use, this PR checks the client whenever a message is enqueued in the outbox.

I tested with the following demo:

import asyncio
import time
from nicegui import app, ui

handlers = []

async def loop():
    while True:
        for handler in handlers:
            handler()
        await asyncio.sleep(1)

app.on_startup(loop)

@ui.page('/')
def page():
    log = ui.log(max_lines=10)
    handlers.append(lambda: log.push(f'Hello at {time.time()}'))

When closing a client and waiting a few seconds, a warning appears that a deleted client is used. Without this PR, we need to wait until elements are deleted from the overflowing log element, which causes a KeyError.