Option to turn off `reevaluate_occupancy`

During the call earlier, we discussed turning off `reevaluate_occupancy` and seeing how things ran. TBH I'm not actually sure how to do this. Looking at the code in a few placed, it seems like `reevaluate_occupancy` gets added to the `IOLoop` (see below), but there doesn't appear to be a clear way to influence whether it is called either by disabling or delaying the frequency of the call. Is there one and I'm just overlooking things? Or should we add one? Related we can disable stealing, which we've played with before, but this only affects things after going through a fair bit of `reevaluate_occupancy`. So not sure if this is what we had in mind. Thoughts? 🙂

https://github.com/dask/distributed/blob/383ea0326ae103b5d5e0b62ed9c3cb18510c5b9e/distributed/scheduler.py#L3273

https://github.com/dask/distributed/blob/383ea0326ae103b5d5e0b62ed9c3cb18510c5b9e/distributed/scheduler.py#L6490-L6492

cc @mrocklin @quasiben