Stratum: Use RwLock for connection_mapping to avoid blocking RPC

 ## Thread-Safe Connection Mapping with RwLock

Wrap connection_mapping in Arc<RwLock<ConnectionMapping>> so that:

• getminerinfo and the Notifier use read locks, preventing Stratum from being blocked when:

  • miners are added/removed
  • job notifications are sent

• Stratum add/remove operations use write locks, keeping the critical sections short.

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Updates

main.rs

• Create connection_mapping using RwLock.

stratum.rs

• Server and Notifier now use read/write locks.
• Tests updated accordingly.

rpc_server.rs

• get_miner_info now uses read().await.
• run_rpc_server and tests updated to use RwLock for the mapping.