One unreachable device delays requests for every other device

[amarnath-ac]

All WSMAN connection setups run on one worker goroutine. A device that can't authenticate holds it while it waits, so other devices queue behind it. Their AMT calls come back very late or time out with 504, even though those devices are reachable.

To Reproduce:

1. Start Console without CIRA.
2. Add a real vPro device that works.
3. Add a dummy device with an unreachable IP.
4. Open the dummy device's details page and leave it loading.
5. Go back to the device list and open the real device's details.
6. The real device shows no data. Its calls return 200 only after a minute or more.
7. The dummy device's calls fail with 504 at 30s, 1m, 1m30s, 2m.

Step 5 has to happen while the dummy device is still loading. Wait for it to finish and everything looks fine, so the bug seems intermittent.

Expected behavior:
An unreachable device should fail on its own without holding up the others.