Move socket write event handling in to the RFB core

What to do when a socket is writeable should be handled in the
RFB core code as there may be other events we want to fire off
when this happens.
diff --git a/common/rfb/VNCSConnectionST.cxx b/common/rfb/VNCSConnectionST.cxx
index 932f579..0f4ca94 100644
--- a/common/rfb/VNCSConnectionST.cxx
+++ b/common/rfb/VNCSConnectionST.cxx
@@ -190,6 +190,17 @@
   }
 }
 
+void VNCSConnectionST::flushSocket()
+{
+  if (state() == RFBSTATE_CLOSING) return;
+  try {
+    setSocketTimeouts();
+    sock->outStream().flush();
+  } catch (rdr::Exception &e) {
+    close(e.str());
+  }
+}
+
 void VNCSConnectionST::pixelBufferChange()
 {
   try {