Improve handling of disconnects and reconnects (#228)

* Check response 0 on disconnect

From recv(): The return value will be 0 when the peer has performed an orderly shutdown

* Add persistent presence and handlers

* Use buffer instead of raw struct

* Clear presence data on shutdown

* Remove CurrentPresence and add boolean instead

This removes the need for having 2 big buffers in favor of using a small boolean

src/connection_unix.cpp

@@ -118,5 +118,8 @@ bool BaseConnection::Read(void* data, size_t length)
         }
         Close();
     }
+    else if (res == 0) {
+        Close();
+    }
     return res == (int)length;
 }