Fix silent drop of return payloads under master backpressure

[dwoz]

When RequestClient._send_recv pulled a (future, message) pair off the queue whose future had already been marked done — i.e. the per-message timeout fired while the entry waited behind a slow master — the pre-send if future.done(): continue short-circuit discarded the message without ever sending it. The minion's caller saw the same SaltReqTimeoutError it sees in 3007.x, but the master never received the frame, so _return / event_return / external job cache side effects never fired and result stores were left silently incomplete. The reproducer in TRANSPORT_BUG.md shows ~4x delivery loss in 3008.0rc3 vs 3007.14 under the same workload.

The short-circuit was added during the worker-pool routing rewrite (#68532) as a load-shedding optimization. That intent is sound for genuine request/response traffic, but wrong for fire-and-await-ACK payloads like _return: the master persists the side effects from the message itself, regardless of whether the originating future is still alive. Remove the pre-send check from both _send_recv implementations. The remaining post-send future.done() handling and the recv-loop's elif future.done(): break preserve the back-pressure guarantees (POLLOUT/POLLIN poll budgets, reconnect on stale futures) — same control flow as 3007.x.

Add a regression test that pre-stages 20 stale-future entries on the queue and asserts every one reaches the mocked wire. Fails on the unpatched source with 0/20 sends recorded.

Also fix test_client_timeout_msg to use a guaranteed-unused local port instead of the default 4506 — the test assumes nothing is listening there, which collides with any host running a real salt-master.