Revert "Delete redundant token2author DB save"

Something's weird here; this change shouldn't have any effect. I'll
have to squint at the code some more.

This reverts commit 2bf076043f.

Fixes #4262

src/node/db/AuthorManager.js

@@ -77,6 +77,17 @@ exports.createAuthorIfNotExistsFor = async function(authorMapper, name)
   return author;
 };
 
+/**
+ * Sets the token <> AuthorID relationship.
+ * Discussion at https://github.com/ether/etherpad-lite/issues/4006
+ * @param {String} token The token (generated by a client)
+ * @param {String} authorID The authorID (returned by the Security Manager)
+ */
+exports.setToken2Author = async function(token, authorID)
+{
+  await db.set("token2author:"+token, authorID);
+}
+
 /**
  * Returns the AuthorID for a mapper. We can map using a mapperkey,
  * so far this is token2author and mapper2author