db/SessionStore.js: do not migrate to Promises. Make optional all(), clear() and length()

1. This module was not migrated to Promises, because it is only used via
   express-session, which can't actually use promises anyway.

2. all(), clear() and length() depend on the presence of the `db.forEach()`
   function, which in ueberdb2 doesn't even exist.

   Fortunately those three methods are optional, so I made their existence
   conditional on the presence of `db.forEach`.

3. in SessionStore.clear(), replaced a call to db.db.remove() with db.remove()

src/node/db/SessionStore.js

@@ -2,6 +2,9 @@
  * Stores session data in the database
  * Source; https://github.com/edy-b/SciFlowWriter/blob/develop/available_plugins/ep_sciflowwriter/db/DirtyStore.js
  * This is not used for authors that are created via the API at current
+ *
+ * RPB: this module was not migrated to Promises, because it is only used via
+ *      express-session, which can't actually use promises anyway.
  */
 
 var Store = require('ep_etherpad-lite/node_modules/express-session').Store,
@@ -50,39 +53,46 @@ SessionStore.prototype.destroy = function(sid, fn) {
   }
 };
 
-SessionStore.prototype.all = function(fn) {
+/*
-  messageLogger.debug('ALL');
+ * RPB: the following methods are optional requirements for a compatible session
+ *      store for express-session, but in any case appear to depend on a
+ *      non-existent feature of ueberdb2
+ */
+if (db.forEach) {
+  SessionStore.prototype.all = function(fn) {
+    messageLogger.debug('ALL');
 
-  var sessions = [];
+    var sessions = [];
 
-  db.forEach(function(key, value) {
+    db.forEach(function(key, value) {
-    if (key.substr(0,15) === "sessionstorage:") {
+      if (key.substr(0,15) === "sessionstorage:") {
-      sessions.push(value);
+        sessions.push(value);
-    }
+      }
-  });
+    });
-  fn(null, sessions);
+    fn(null, sessions);
-};
+  };
-
+
-SessionStore.prototype.clear = function(fn) {
+  SessionStore.prototype.clear = function(fn) {
-  messageLogger.debug('CLEAR');
+    messageLogger.debug('CLEAR');
-
+
-  db.forEach(function(key, value) {
+    db.forEach(function(key, value) {
-    if (key.substr(0,15) === "sessionstorage:") {
+      if (key.substr(0,15) === "sessionstorage:") {
-      db.db.remove("session:" + key);
+        db.remove("session:" + key);
-    }
+      }
-  });
+    });
-  if (fn) fn();
+    if (fn) fn();
-};
+  };
-
+
-SessionStore.prototype.length = function(fn) {
+  SessionStore.prototype.length = function(fn) {
-  messageLogger.debug('LENGTH');
+    messageLogger.debug('LENGTH');
-
+
-  var i = 0;
+    var i = 0;
-
+
-  db.forEach(function(key, value) {
+    db.forEach(function(key, value) {
-    if (key.substr(0,15) === "sessionstorage:") {
+      if (key.substr(0,15) === "sessionstorage:") {
-      i++;
+        i++;
-    }
+      }
-  });
+    });
-  fn(null, i);
+    fn(null, i);
+  }
 };