Make StdSchedulerFactory and ServiceCollectionSchedulerFactories threadsafe

[jcracknell]

fixes #1587

I moved the call to SchedulerRepository.Bind to GetScheduler, as this makes the registration process clearer.

You can't write tests against this as it would pollute the SchedulerRepository singleton as noted in #1453.

[jcracknell]

I thought about this some more last night and there's a second bug here where the ServiceCollectionSchedulerFactory would fail to initialize a new scheduler instance produced by StdSchedulerFactory in the event that the previously initialized scheduler was shut down:

quartznet/src/Quartz/Impl/StdSchedulerFactory.cs

Lines 1103 to 1119 in 2a5e43d

	IScheduler? sched = await schedRep.Lookup(SchedulerName, cancellationToken).ConfigureAwait(false);
	if (sched != null)
	{
	if (sched.IsShutdown)
	{
	schedRep.Remove(SchedulerName);
	}
	else
	{
	return sched;
	}
	}
	sched = await Instantiate().ConfigureAwait(false);
	return sched!;

[lahma]

Looks good, thank you!

[lahma]

@jcracknell I hate to ask, but would you be willing to create another PR against main as this doesn't merge cleanly due to some structural changes done there.