Use timer instead of context for query no poller wait

[dnr]

What changed?

Tweak the "return special error if no recent poller when we're near the deadline" logic:

• Use a one-shot timer instead of a context.
• Move it into the loop so that it also can activate in the errForwarderSlowDown case.

Why?

• Context Done channels are closed, so it becomes a busy loop after close. A timer is more efficient here anyway.
• Use the logic in more potential situations for better error messages.

How did you test it?

Existing tests. The previous code had correct behavior besides the busy loop.