Attempting to lock a nonexistent item hangs indefinitely

[hategan]

If /dm/lock is called with an invalid item id, the call hangs. That's because waitForPendingDelete waits until a query that searches for an item with the relevant id AND dm.deleteInProgress not equal to true succeeds. /dm/lock should validate the item id before calling waitForPendingDelete or waitForPendingDelete should fail if the item with the specified id does not exist.