Don't create job output datasets on disk in web threads

[jmchilton]

The job setup code in the job handler should be doing the makedirs and stuff but it appears to happen in job creation code in response to tool form submission. This means a slower user experience and also means disk slow downs could prevent jobs from being created and hinder user experience even further and probably hang web threads as well. After #6373 is merged, this functionality is called from ObjectStorePopulator.create_datasets() in galaxy.tools.actions but it really needs to be moved to the job handler I think (#6373 doesn't make this worse - it actually separates things out a little better and hopefully will allow us to improve this more easily).

xref #4959