[BUG] - Data Race between scheduler stopping a job and stopping the scheduler

[ryanrazsa]

Describe the bug

If a job is removed by the scheduler due to a condition (ex max runs is hit), and the scheduler is stopped, a data race occurs.

To Reproduce

Steps to reproduce the behavior:

1. Schedule a job with a max number of runs
2. Wait for the job to hit the max runs
3. Stop the scheduler

To reproduce in test code

func TestDataRace(t *testing.T) {
	sut := gocron.NewScheduler(time.UTC)
	sut.StartAsync()
	maxRuns := 10
	task := func() {
	}

	_, err := sut.
		Every(1).
		Milliseconds().
		Tag("name").
		SingletonMode().
		LimitRunsTo(maxRuns).
		Do(task)
	assert.NoError(t, err)

	time.Sleep(1 * time.Second)
	sut.Stop()
}

go test -race ./...

=== RUN   TestDataRace
==================
WARNING: DATA RACE
Read at 0x00c000192018 by goroutine 6:
  github.com/go-co-op/gocron.(*Scheduler).stop()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:895 +0x30
  github.com/go-co-op/gocron.(*Scheduler).Stop()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:890 +0x50
  github.com/StackAdapt/go-user-writer/internal/scheduler.TestDataRace()
      scheduler_test.go:140 +0x100
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1576 +0x188
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1629 +0x40

Previous write at 0x00c000192018 by goroutine 31:
  github.com/go-co-op/gocron.(*Scheduler).setJobs()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:147 +0x78
  github.com/go-co-op/gocron.(*Scheduler).removeByCondition()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:728 +0x18c
  github.com/go-co-op/gocron.(*Scheduler).RemoveByReference()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:692 +0x54
  github.com/go-co-op/gocron.(*Scheduler).scheduleNextRun()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:222 +0x614
  github.com/go-co-op/gocron.(*Scheduler).runContinuous()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:599 +0x34
  github.com/go-co-op/gocron.(*Scheduler).runContinuous.func1()
      github.com/go-co-op/gocron@v1.31.1/scheduler.go:632 +0x238

Version

v1.31.1
go1.20.3

Expected behavior

Possibly expecting stopJobs to use the jobsMutex

Additional context

I don't know the repo well enough to know the implications of this fix, but I created a PR as an example of how it can be fixed

#540