gnoi-shutdown: use per-thread DB connections to avoid crossed DPU reads

[gpunathilell]

What I did

gnoi-shutdown-daemon spawns one worker thread per DPU on shutdown, but all threads shared a single CONFIG_DB/STATE_DB redis connection created once in main(). A redis DBConnector is a single, non-thread-safe socket, so when multiple DPUs shut down in parallel their hget/Table reads interleave on the wire and replies get matched to the wrong request — producing crossed IP/port values between DPUs.

This gave gnoi_client a bad target like 169.254.200.1:169.254.200.2 (dpu0's IP as host, dpu1's IP in the port slot), causing:

ERR gnoi-shutdown-daemon: DPU1: Reboot command failed (rc=2, target=169.254.200.1:169.254.200.2):
panic: rpc error: ... dial tcp: lookup tcp/169.254.200.2: unknown port

Fix

Give each per-DPU worker thread its own CONFIG_DB/STATE_DB connection so reads can never cross:

• handle_and_cleanup() opens fresh db_connect("CONFIG_DB") / db_connect("STATE_DB") per thread and passes them into _handle_transition().
• _handle_transition() / _wait_for_gnoi_halt_in_progress() take optional per-thread connections, falling back to the shared ones for direct/unit-test callers (backward compatible).

Testing

• py_compile clean.
• Smoke-tested both paths: per-thread connections are the ones actually used (correct 169.254.200.2:50052), and fallback-to-shared preserves existing unit-test behavior.

[mssonicbld]

/azp run

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