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,
@@ -50,6 +53,12 @@ SessionStore.prototype.destroy = function(sid, fn) {
   }
 };
 
+/*
+ * 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');
 
@@ -68,7 +77,7 @@ SessionStore.prototype.clear = function(fn) {
 
     db.forEach(function(key, value) {
       if (key.substr(0,15) === "sessionstorage:") {
-      db.db.remove("session:" + key);
+        db.remove("session:" + key);
       }
     });
     if (fn) fn();
@@ -85,4 +94,5 @@ SessionStore.prototype.length = function(fn) {
       }
     });
     fn(null, i);
+  }
 };