Refactor stratum server leak handling

[syntaxjak]

Addressing issue: #3867

• Reworked the Stratum connection handling so reads and writes can run independently
• Added bounded response queues, connection limits, and idle timeouts
• Added an RAII cleanup guard so workers are removed correctly on disconnects and errors
• Stratum connections and listener tasks now stop cleanly when the node shuts down
• Added tests for connection cleanup, limits, backpressure, idle handling, and shutdown
• Removed the unused async-stream dependency

[iho]

Heads-up: #3889 also targets #3867 with the same structural approach (handle_connection + Drop guard + simpler accept loop).

That PR additionally adds idle timeout, max workers, and bounded write queues for the half-open / reconnect-storm failure mode. Worth coordinating so the two don’t land overlapping refactors without combining the resource bounds.

[wiesche89]

I tested this with a miner and additional connection tests. The cleanup works for normal disconnects, but it does not fully solve the reported connection leak yet. I’ve left a few comments on the remaining cases.

https://github.com/GetGrin/grim/issues/19