Check staging locations

For every file ref in foxml that starts with "tag:", check that we recognize the staging area and that we can connect to it. Tries to connect the staging area twice, a second time after 5 second delay. This is to see if the first access attempt wakes the mount point, since we've seen it do that. Once a staging area is connected, it doesn't get polled again. (Stages object stored connected state.)
The goal is not to check all the file references, only to make sure that each referenced staging area is mounted. So for that purpose "tag" is effectively it. (IRODS being deprecated for staging anything presently)