Disposable issues

[adamradocz]

Noticed a few potential issues:

• ConnectionLimitMiddleware doesn't call Dispose on SemaphoreSlim _limiter.
• ConsumableArrayBufferWriter should be sealed or the Dispose should be virtual.
• TimerAwaitable should be sealed or the Dispose should be virtual.
• ConnectionContextWithDelegate is disposing the ConnectionContext connection, although it is injected.
• SocketSender doesn't call Dispose on SocketAsyncEventArgs _eventArgs. Plus, the _eventArgs.Completed += (_, e) => ((SocketAwaitable)e.UserToken).Complete(e.BytesTransferred, e.SocketError); not a memory leak, as it never unsubscribed? This is the same with the SocketReceiver.
• ServerConnection doesn't call Dispose on CancellationTokenSource _connectionClosingCts.
• Server doesn't call Dispose on TimerAwaitable _timerAwaitable.