Fix event loop freeze: replace blocking stratum write with async queue

send_response() used synchronous boost::asio::write() on the ioc thread.
When a miner's TCP receive buffer was full (network issue), this blocked
the entire single-threaded event loop — no timers, P2P, or watchdog
could fire until the kernel TCP retransmission timed out (~15 min).

Replace with async_write + deque write queue pattern. Each send_response()
pushes to the queue and kicks do_write() which chains async_write calls.
On error, the socket is closed and the queue cleared.

Also adds runtime LevelDB pruning: batch-delete shares evicted by
clean_tracker's drop-tails step, matching p2pool's tracker.removed.watch
pattern that propagates in-memory removal to disk storage.

Author: frstrtr

src/core/stratum_server.cpp

@@ -1085,12 +1085,36 @@ void StratumSession::send_response(const nlohmann::json& response)
 {
     try {
         std::string message = response.dump() + "\n";
-        boost::asio::write(socket_, boost::asio::buffer(message));
+        write_queue_.push_back(std::move(message));
+        if (!writing_)
+            do_write();
     } catch (const std::exception& e) {
-        LOG_ERROR << "[Stratum] Error sending response: " << e.what();
+        LOG_ERROR << "[Stratum] Error queueing response: " << e.what();
     }
 }
 
+void StratumSession::do_write()
+{
+    if (write_queue_.empty() || !socket_.is_open()) {
+        writing_ = false;
+        return;
+    }
+    writing_ = true;
+    auto self = shared_from_this();
+    boost::asio::async_write(socket_, boost::asio::buffer(write_queue_.front()),
+        [this, self](boost::system::error_code ec, std::size_t /*bytes*/) {
+            if (ec) {
+                LOG_ERROR << "[Stratum] Error sending response: " << ec.message();
+                write_queue_.clear();
+                writing_ = false;
+                try { socket_.close(); } catch (...) {}
+                return;
+            }
+            write_queue_.pop_front();
+            do_write();
+        });
+}
+
 void StratumSession::send_error(int code, const std::string& message, const nlohmann::json& request_id)
 {
     nlohmann::json response;

src/core/stratum_server.hpp

@@ -87,6 +87,8 @@ class StratumSession : public std::enable_shared_from_this<StratumSession>
 {
     tcp::socket socket_;
     boost::asio::streambuf buffer_;
+    std::deque<std::string> write_queue_;  // async write queue (non-blocking)
+    bool writing_ = false;                 // true while an async_write is in flight
     std::shared_ptr<MiningInterface> mining_interface_;
     StratumServer* server_ = nullptr;  // back-pointer for RateMonitor recording
     std::string subscription_id_;
@@ -192,6 +194,7 @@ class StratumSession : public std::enable_shared_from_this<StratumSession>
     nlohmann::json handle_suggest_difficulty(const nlohmann::json& params, const nlohmann::json& request_id);
 
     void send_response(const nlohmann::json& response);
+    void do_write();  // drain the async write queue
     void send_error(int code, const std::string& message, const nlohmann::json& request_id);
     void send_set_difficulty(double difficulty);
     void send_set_extranonce(const std::string& extranonce1, int extranonce2_size);