The stageDebug command should:

• be able to run on secondaries (unless a write operation is requested)
• acquire the appropriate collection lock in IS/IX mode
• run with a yielding PlanExecutor
• support the "update" stage (and other stages that may be missing)
• possibly not automatically add a FETCH stage on top of the user-requested stage tree

We should also audit jstests/core/stages_*.js to ensure they provide the coverage we're looking for, and verify that the FSM suite is indeed able to run them in parallel after the collection lock changes are made.

(Or, as an alternative to the above work, we could outright remove the stageDebug command and associated tests.)