Cron - Not running multiple instances of a job

還沒有人認領這個 Issue。

This is not an elaborate solution, but it works (in theory). You can't choose to have multiple instances running or not. I just assumed not to have multiple instances running regardless. I assumed this is not required unless you are running a job every minute or two.

I added a field to the scheduler that tracks the `Task` returned by `job.Run(stoppingToken);`. On ever tick check the list for completed tasks and clear them. Prior to running a job skip if it is on the list of running jobs.

I'm sure this might need some tweaking this was my first attempt at it.

```
private Dictionary _runningJobs;

.
.
.

// Clear completed jobs
foreach (var key in _runningJobs.Keys)
{
if (_runningJobs[key].IsCompleted)
{
_runningJobs[key].Dispose();
_runningJobs.Remove(key);
}
}

// Run jobs that are in the map
RunActiveJobs(runMap, now, stoppingToken);

.
.
.

foreach (var run in currentRuns)
{

// We are sure (thanks to our extension method)
// that the service is of type ICronJob
var job = (ICronJob)_serviceProvider.GetRequiredService(run);
string jobName = job.GetType().Name;

// Continue to next job if job is already running
if (_runningJobs.ContainsKey(jobName))
{
continue;
}

// We don't want to await jobs explicitly because that
// could interfere with other job runs
var task = job.Run(stoppingToken);

// Add task to running jobs
_runningJobs.Add(jobName, task);
}
```

首先定位 scheduler tick 的處理邏輯、ICronJob 進入點以及對 job.Run(stoppingToken) 的呼叫。追蹤排程工作如何被選取和追蹤。完成標準是：排程工作在其上一次 task 仍在執行時不會再次啟動，但在完成後可以啟動後續執行。

由索引模型根據 Issue 內容生成。