lib: implement mu_store_update

lib/mu-store.hh

@@ -122,6 +122,16 @@ public:
          */
         unsigned add_message (const std::string& path);
 
+        /**
+         * Update a message in the store.
+         *
+         * @param msg a message
+         * @param docid a docid
+         *
+         * @return false in case of failure; true ottherwise.
+         */
+        bool update_message (MuMsg *msg, unsigned docid);
+
         /**
          * Add a message to the store.
          *