Scope "in-flight" PHIDs more carefully

Fixes T8125. In Feed, we query for a bunch of objects and also a bunch of transactions.

The transactions require the objects. Normally, whichever executes last will fill out of the Workspace cheaply, so this query strategy is fine overall.

The new "in-flight" code would mark everything in flight before the transactions loaded, though, so they'd fail to load even though the query plan is not cyclic.

Instead, be more surgical and mark things in flight only immediately before we put them in flight.

Test Plan: Feed now shows more stories again; files with cycles still load in finite time.

Auditors: btrahan