Items being imported are considered stale objects in discovery

[benbosman]

Describe the bug
During an import, items added to discovery can be deleted again before the import script completed

To Reproduce
Steps to reproduce the behavior:

• Start a large import (using scripts and processes or a command line script)
• Wait for it to have processed the DB changes, and starts the consumers (discovery indexing)
• Don't wait so long that the script completed (and the DB changes are committed)
• Perform a discovery query in the UI (or REST) containing items which were indexed
• The logs will mention Cleaning x stale objects from Discovery Index
• The records will no longer be in discovery
• Later, the script completes, and the DB changes are committed, but the records are already gone from Discovery

Expected behavior
Items which are not deleted, but rather still being imported, should not removed from discovery

Alternatively, the DB changes should be committed before the discovery updates can start

Related work
#3265