Improved CRC32Map checking in several ways:

- servers now compute the map in a separate thread
- CRC32Maps are kept in cache for 30 mins
- CRC32Maps requests cannot be used to overflood a server anymore since their number is limited.
- Transfer modules now send keep alive packets to maintain tunnels when asking for a CRC32Map



git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@4661 b45a01b8-16f6-495d-af2f-9b41ad6348cc

libretroshare/src/ft/fttransfermodule.h

@@ -193,6 +193,8 @@ private:
   CRC32Map _crcmap ;
   uint32_t _crcmap_state ;
   time_t   _crcmap_last_asked_time ;
+  time_t   _crcmap_last_tunnel_keepup ;
+  std::string _crcreq_source ;
 
   ftFileStatus mFileStatus; //used for pause/resume file transfer