Try to fix memory leaks by clear OT server before disconnect note

Author: jackycute

lib/realtime.js

@@ -543,6 +543,9 @@ function disconnect(socket) {
             if (note.server.isDirty) {
                 updateNote(note, function (err, _note) {
                     if (err) return logger.error('disconnect note failed: ' + err);
+                    // clear server before delete to avoid memory leaks
+                    note.server.document = "";
+                    note.server.operations = [];
                     delete note.server;
                     delete notes[noteId];
                     if (config.debug) {