使用SemaphoreSlim控制并发数

`static readonly SemaphoreSlim _lock = new SemaphoreSlim(2);

public async Task TestSlim(string name)
{
if (!await _lock.WaitAsync(0))
{
return Content($"劳资很忙，晚点再来！");
}
try
{
await Task.Delay(5000);
return Content($"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff}--->任务{name}完成了...");
}
finally
{
_lock.Release();
}
}`