use defines for values in source mask of message key and simplified a bit (in preparation of #278), documentation

src/lib/ebus/message.h

@@ -1410,6 +1410,17 @@ class MessageMap : public MappedFileReader {
     bool completeMatch, bool withRead, bool withWrite, bool withPassive, bool includeEmptyLevel, bool onlyAvailable,
     time_t since, time_t until, bool changedSince, deque<Message*>* messages) const;
 
+  /**
+   * Get the first available @a Message from the first map iterator entry.
+   * @param it the map iterator with list of @a Message instances to check.
+   * @param sameIdExtAs the optional @a MasterSymbolString to check for having the same ID.
+   * @param onlyAvailable true to include only available messages (default true), false to also include messages that
+   * are currently not available (e.g. due to unresolved or false conditions).
+   * @return the first available @a Message from the first map iterator entry.
+   */
+  Message* getFirstAvailableFromIterator(const map<uint64_t, vector<Message*> >::const_iterator& it,
+    const MasterSymbolString* sameIdExtAs, bool onlyAvailable) const;
+
   /**
    * Find the @a Message instance for the specified master data.
    * @param master the @a MasterSymbolString for identifying the @a Message.