Replace all uses of @async with Threads.@spawn

[JamesWrigley]

- Use errormonitor_tracked() for executing DTask's
- Set allow_errors=false by default in the eager scheduler.

I don't know why we had allow_errors=true originally but it seems way too permissive, it caused some internal exceptions to not be logged when I was working on the streaming branch.

[JamesWrigley]

In hindsight I should've run the tests locally first 😆 I'll try to fix those errors.

[JamesWrigley]

Using errormonitor_tracked() would still print out some ugly error messages when I don't think we want it to so I decided to revert that, now the only change is to use Threads.@spawn everywhere instead of @async.

[jpsamaroo]

Thanks!