http: prevent race condition between worker thread and I/O thread

This prevents a losing race condition that could prevent the server
from reading requests from an HTTP client.

A connected socket can either be written to or read from based on the
result of GenerateWaitSockets(). That method checks the HTTPRemoteClient
flag m_send_ready. If it's `true` the implication is that there is
data in the client's send buffer ready to go. Once that data is sent
and the buffer is empty, MaybeSendBytesFromBuffer() sets it `false` again.

The sad case was when a worker thread calling WriteReply() adds
data to the send buffer, but before it sets m_send_ready to `true`,
the I/O thread sends that data and empties the buffer. With the
buffer unexpectedly empty, WriteReply() sets m_send_ready to `true`.

The effect of this is that the socket will stay in "write" mode
with nothing to write. With nothing to write, MaybeSendBytesFromBuffer()
never sets it back to `false` and the socket is stuck forever.

src/httpserver.h

@@ -483,11 +483,14 @@ public:
     /// @}
 
     /**
-     * Set true by worker threads after writing a response to m_send_buffer.
-     * Set false by the HTTPServer I/O thread after flushing m_send_buffer.
-     * Checked in the HTTPServer I/O loop to avoid locking m_send_mutex if there's nothing to send.
-     */
-    std::atomic_bool m_send_ready{false};
+    * Set true by worker threads after writing a response to m_send_buffer.
+    * Set false by the HTTPServer I/O thread after flushing m_send_buffer.
+    * Checked in the HTTPServer I/O loop to decide whether to poll the socket for
+    * writeability or readability.
+    * Guarded by m_send_mutex so it stays consistent with m_send_buffer's emptiness:
+    * the two must always be updated together under the same lock.
+    */
+    bool m_send_ready GUARDED_BY(m_send_mutex){false};
 
     /**
      * Mutex that serializes the Send() and Recv() calls on `m_sock`. Reading